01_Data_Visualization_DataVisualizationWithD3.md

Introduction to the Data Visualization with D3

D3.js, or D3, stands for Data Driven Documents. D3 is a JavaScript library to create dynamic and interactive data visualizations in the browser. It's built to work with common web standards, namely HTML, CSS, and Scalable Vector Graphics (SVG).
D3 takes input data and maps it into a visual representation of that data. It supports many different data formats. D3 lets you bind (or attach) the data to the Document Object Model (DOM). You use HTML or SVG elements with D3's built-in methods to transform the data into a visualization.

Add Document Elements with D3

D3 has several methods that let you add and change elements in your document.
The select() method selects one element from the document. It takes an argument for the name of the element you want and returns an HTML node for the first element in the document that matches the name. Here's an example: const anchor = d3.select("a");
The above example finds the first anchor tag on the page and saves an HTML node for it in the variable anchor. You can use the selection with other methods. The "d3" part of the example is a reference to the D3 object, which is how you access D3 methods.
Two other useful methods are append() and text().
- The append() method takes an argument for the element you want to add to the document. It appends an HTML node to a selected item, and returns a handle to that node.
- The text() method either sets the text of the selected node, or gets the current text. To set the value, you pass a string as an argument inside the parentheses of the method.
D3 allows you to chain several methods together with periods to perform a number of actions in a row.
Example: Use the select method to select the body tag in the document. Then append an h1 tag to it, and add the text "Learning D3" into the h1 element.

<body>
  <script>
    d3.select("body")
      .append("h1")
      .text("Learning D3")
  </script>
</body>

Select a Group of Elements with D3

D3 also has the selectAll() method to select a group of elements. It returns an array of HTML nodes for all the items in the document that match the input string. Here's an example to select all the anchor tags in a document:

const anchors = d3.selectAll("a");

Like the select() method, selectAll() supports method chaining, and you can use it with other methods.
Example: Select all of the li tags in the document, and change their text to "list item" by chaining the .text() method.

<body>
  <ul>
    <li>Example</li>
    <li>Example</li>
    <li>Example</li>
  </ul>
  <script>
    d3.selectAll("li")
      .text("list item")
  </script>
</body>

Work with Data in D3

The D3 library focuses on a data-driven approach. When you have a set of data, you can apply D3 methods to display it on the page. Data comes in many formats, but this challenge uses a simple array of numbers.
The first step is to make D3 aware of the data. The data() method is used on a selection of DOM elements to attach the data to those elements. The data set is passed as an argument to the method.
A common workflow pattern is to create a new element in the document for each piece of data in the set. D3 has the enter() method for this purpose.
When enter() is combined with the data() method, it looks at the selected elements from the page and compares them to the number of data items in the set. If there are fewer elements than data items, it creates the missing elements.
Example: Select the body node, then select all h2 elements. Have D3 create and append an h2 tag for each item in the dataset array. The text in the h2 should say "New Title". Your code should use the data() and enter() methods.

<body>
  <script>
    const dataset = [12, 31, 22, 17, 25, 18, 29, 14, 9];
    d3.select("body")
      .selectAll("h2")
      .data(dataset)
      .enter()
      .append("h2")
      .text("New Title")
  </script>
</body>

Work with Dynamic Data in D3

The D3 text() method can take a string or a callback function as an argument:

selection.text((d) => d)

In the example above, the parameter d refers to a single entry in the dataset that a selection is bound to.
Example: Change the text() method so that each h2 element displays the corresponding value from the dataset array with a single space and "USD". For example, the first heading should be "12 USD".

Soulution 1:

<body>
  <script>
    const dataset = [12, 31, 22, 17, 25, 18, 29, 14, 9];

    d3.select("body").selectAll("h2")
      .data(dataset)
      .enter()
      .append("h2")
      .text(d => d + " USD");
      
  </script>
</body>

Solution 2:

<body>
  <script>
    const dataset = [12, 31, 22, 17, 25, 18, 29, 14, 9];
    
    d3.select("body").selectAll("h2")
      .data(dataset)
      .enter()
      .append("h2")
      .text((d) => `${d} USD`);
      
  </script>
</body>

Add Inline Styling to Elements

D3 lets you add inline CSS styles on dynamic elements with the style() method.
The style() method takes a comma-separated key-value pair as an argument. Here's an example to set the selection's text color to blue:

selection.style("color","blue");

Example: Add the style() method to the code in the editor to make all the displayed text have a font-family of verdana.

<body>
  <script>
    const dataset = [12, 31, 22, 17, 25, 18, 29, 14, 9];

    d3.select("body").selectAll("h2")
      .data(dataset)
      .enter()
      .append("h2")
      .text((d) => (d + " USD"))
      .style("font-family","verdana")
      
  </script>
</body>

Change Styles Based on Data

D3 is about visualization and presentation of data. It's likely you'll want to change the styling of elements based on the data. You can use a callback function in the style() method to change the styling for different elements.
eg.

selection.style("color", (d) => {
  /* Logic that returns the color based on a condition */
});

The style() method is not limited to setting the color - it can be used with other CSS properties as well.
Example: Add the style() method to the code in the editor to set the color of the h2 elements conditionally. Write the callback function so if the data value is less than 20, it returns "red", otherwise it returns "green".

<body>
  <script>
    const dataset = [12, 31, 22, 17, 25, 18, 29, 14, 9];

    d3.select("body").selectAll("h2")
      .data(dataset)
      .enter()
      .append("h2")
      .text((d) => (d + " USD"))
      .style("color", d => {
        if (d < 20) {
          return "red"
        } else {
          return "green"
        }
      })
  </script>
</body>

Add Classes with D3

Using a lot of inline styles on HTML elements gets hard to manage, even for smaller apps. It's easier to add a class to elements and style that class one time using CSS rules. D3 has the attr() method to add any HTML attribute to an element, including a class name.
The attr() method works the same way that style() does. It takes comma-separated values, and can use a callback function. Here's an example to add a class of "container" to a selection:

selection.attr("class", "container");

Example: Add the attr() method to the code in the editor and put a class of bar on the div elements.

<style>
  .bar {
    width: 25px;
    height: 100px;
    display: inline-block;
    background-color: blue;
  }
</style>
<body>
  <script>
    const dataset = [12, 31, 22, 17, 25, 18, 29, 14, 9];

    d3.select("body").selectAll("div")
      .data(dataset)
      .enter()
      .append("div")
      .attr("class","bar")
    
  </script>
</body>

Update the Height of an Element Dynamically

Recall the format to set a style using a callback function:

selection.style("cssProperty", (d) => d)

Exaple: Add the style() method to the code in the editor to set the height property for each element. Use a callback function to return the value of the data point with the string "px" added to it.

<style>
  .bar {
    width: 25px;
    height: 100px;
    display: inline-block;
    background-color: blue;
  }
</style>
<body>
  <script>
    const dataset = [12, 31, 22, 17, 25, 18, 29, 14, 9];

    d3.select("body").selectAll("div")
      .data(dataset)
      .enter()
      .append("div")
      .attr("class", "bar")
      .style("height", d => d + 'px')
      // or style("height", d => `${d}px`)
  </script>
</body>

Change the Presentation of a Bar Chart

The last challenge created a bar chart, but there are a couple of formatting changes that could improve it:
1. Add space between each bar to visually separate them, which is done by adding a margin to the CSS for the bar class
1. Increase the height of the bars to better show the difference in values, which is done by multiplying the value by a number to scale the height
Example:
- First, add a margin of 2px to the bar class in the style tag. Next, change the callback function in the style() method so it returns a value 10 times the original data value (plus the "px").
- Note: Multiplying each data point by the same constant only alters the scale. It's like zooming in, and it doesn't change the meaning of the underlying data.

d3.select("body").selectAll("div")
  .data(dataset)
  .enter()
  .append("div")
  .attr("class", "bar")
  .style("height", (d) => (d*10 + "px"))

</script>

Learn About SVG in D3

SVG stands for Scalable Vector Graphics.
Here "scalable" means that, if you zoom in or out on an object, it would not appear pixelated. It scales with the display system, whether it's on a small mobile screen or a large TV monitor.
SVG is used to make common geometric shapes. Since D3 maps data into a visual representation, it uses SVG to create the shapes for the visualization. SVG shapes for a web page must go within an HTML svg tag.
CSS can be scalable when styles use relative units (such as vh, vw, or percentages), but using SVG is more flexible to build data visualizations.
Example:
- Add an svg node to the body using append(). Give it a width attribute set to the provided w constant and a height attribute set to the provided h constant using the attr() or style() methods for each. You'll see it in the output because there's a background-color of pink applied to it in the style tag.
- Note: When using attr() width and height attributes do not have units. This is the building block of scaling - the element will always have a 5:1 width to height ratio, no matter what the zoom level is.

<style>
  svg {
    background-color: pink;
  }
</style>
<body>
  <script>
    const dataset = [12, 31, 22, 17, 25, 18, 29, 14, 9];
    const w = 500;
    const h = 100;
    const svg = d3.select("body")
                  .append("svg")
                  .attr("width",w)
                  .attr("height",h)
  </script>
</body>

Display Shapes with SVG

The last challenge created an svg element with a given width and height, which was visible because it had a background-color applied to it in the style tag. The code made space for the given width and height.
The next step is to create a shape to put in the svg area. There are a number of supported shapes in SVG, such as rectangles and circles. They are used to display data. For example, a rectangle (<rect>) SVG shape could create a bar in a bar chart.
When you place a shape into the svg area, you can specify where it goes with x and y coordinates. The origin point of (0, 0) is in the upper-left corner. Positive values for x push the shape to the right, and positive values for y push the shape down from the origin point.
To place a shape in the middle of the 500 (width) x 100 (height) svg from last challenge, the x coordinate would be 250 and the y coordinate would be 50.
An SVG rect has four attributes. There are the x and y coordinates for where it is placed in the svg area. It also has a height and width to specify the size.
Example: Add a rect shape to the svg using append(), and give it a width attribute of 25 and height attribute of 100. Also, give the rect x and y attributes each set to 0.

<body>
  <script>
    const dataset = [12, 31, 22, 17, 25, 18, 29, 14, 9];

    const w = 500;
    const h = 100;

    const svg = d3.select("body")
                  .append("svg")
                  .attr("width", w)
                  .attr("height", h)
                  .append("rect")
                  .attr("width", 25)
                  .attr("height", 100)
                  .attr("x", 0)
                  .attr("y", 0)
  </script>
</body>

Create a Bar for Each Data Point in the Set

Example: Use the data(), enter(), and append() methods to create and append a rect for each item in dataset. The bars should display all on top of each other, this will be fixed in the next challenge.

<body>
  <script>
    const dataset = [12, 31, 22, 17, 25, 18, 29, 14, 9];

    const w = 500;
    const h = 100;

    const svg = d3.select("body")
                  .append("svg")
                  .attr("width", w)
                  .attr("height", h);

    svg.selectAll("rect")
        .data(dataset)
        .enter()
        .append("rect")
       .attr("x", 0)
       .attr("y", 0)
       .attr("width", 25)
       .attr("height", 100);
  </script>
</body>

Dynamically Set the Coordinates for Each Bar

The last challenge created and appended a rectangle to the svg element for each point in dataset to represent a bar. Unfortunately, they were all stacked on top of each other.
The placement of a rectangle is handled by the x and y attributes. They tell D3 where to start drawing the shape in the svg area. The last challenge set them each to 0, so every bar was placed in the upper-left corner.
For a bar chart, all of the bars should sit on the same vertical level, which means the y value stays the same (at 0) for all bars. The x value, however, needs to change as you add new bars. Remember that larger x values push items farther to the right. As you go through the array elements in dataset, the x value should increase.
The attr() method in D3 accepts a callback function to dynamically set that attribute. The callback function takes two arguments, one for the data point itself (usually d) and one for the index of the data point in the array. The second argument for the index is optional. Here's the format:

selection.attr("property", (d, i) => {
  /* 
  * d is the data point value
  * i is the index of the data point in the array
  */
})

It's important to note that you do NOT need to write a for loop or use forEach() to iterate over the items in the data set. Recall that the data() method parses the data set, and any method that's chained after data() is run once for each item in the data set.
Example:
- Change the x attribute callback function so it returns the index times 30.
- Note: Each bar has a width of 25, so increasing each x value by 30 adds some space between the bars. Any value greater than 25 would work in this example.

<body>
  <script>
    const dataset = [12, 31, 22, 17, 25, 18, 29, 14, 9];

    const w = 500;
    const h = 100;

    const svg = d3.select("body")
                  .append("svg")
                  .attr("width", w)
                  .attr("height", h);

    svg.selectAll("rect")
       .data(dataset)
       .enter()
       .append("rect")
       .attr("x", (d, i) => {
          return i*30
       })
       .attr("y", 0)
       .attr("width", 25)
       .attr("height", 100);
  </script>
</body>

Dynamically Change the Height of Each Bar

Example:
- Change the callback function for the height attribute to return the data value times 3.
- Note: Remember that multiplying all data points by the same constant scales the data (like zooming in). It helps to see the differences between bar values in this example.

<body>
  <script>
    const dataset = [12, 31, 22, 17, 25, 18, 29, 14, 9];

    const w = 500;
    const h = 100;

    const svg = d3.select("body")
                  .append("svg")
                  .attr("width", w)
                  .attr("height", h);

    svg.selectAll("rect")
       .data(dataset)
       .enter()
       .append("rect")
       .attr("x", (d, i) => i * 30)
       .attr("y", 0)
       .attr("width", 25)
       .attr("height", (d, i) => {
         return d*3
       });
  </script>
</body>

Invert SVG Elements

You may have noticed the bar chart looked like it's upside-down, or inverted. This is because of how SVG uses (x, y) coordinates.
In SVG, the origin point for the coordinates is in the upper-left corner. An x coordinate of 0 places a shape on the left edge of the SVG area. A y coordinate of 0 places a shape on the top edge of the SVG area. Higher x values push the rectangle to the right. Higher y values push the rectangle down.
To make the bars right-side-up, you need to change the way the y coordinate is calculated. It needs to account for both the height of the bar and the total height of the SVG area.
The y coordinate that is y = heightOfSVG - heightOfBar would place the bars right-side-up.
Example:
- Change the callback function for the y attribute to set the bars right-side-up. Remember that the height of the bar is 3 times the data value d.
- Note: In general, the relationship is y = h - m * d, where m is the constant that scales the data points.

<body>
  <script>
    const dataset = [12, 31, 22, 17, 25, 18, 29, 14, 9];

    const w = 500;
    const h = 100;

    const svg = d3.select("body")
                  .append("svg")
                  .attr("width", w)
                  .attr("height", h);

    svg.selectAll("rect")
       .data(dataset)
       .enter()
       .append("rect")
       .attr("x", (d, i) => i * 30)
       .attr("y", (d, i) => { return (h-d*3); })
       .attr("width", 25)
       .attr("height", (d, i) => 3 * d);
  </script>
</body>

Change the Color of an SVG Element

In SVG, a rect shape is colored with the fill attribute. It supports hex codes, color names, and rgb values, as well as more complex options like gradients and transparency.
Example: Add an attr() method to set the "fill" of all the bars to the color "navy".

<body>
  <script>
    const dataset = [12, 31, 22, 17, 25, 18, 29, 14, 9];

    const w = 500;
    const h = 100;

    const svg = d3.select("body")
                  .append("svg")
                  .attr("width", w)
                  .attr("height", h);

    svg.selectAll("rect")
       .data(dataset)
       .enter()
       .append("rect")
       .attr("x", (d, i) => i * 30)
       .attr("y", (d, i) => h - 3 * d)
       .attr("width", 25)
       .attr("height", (d, i) => 3 * d)
       .attr("fill","navy")
  </script>
</body>

Add Labels to D3 Elements

D3 lets you label a graph element, such as a bar, using the SVG text element.
Like the rect element, a text element needs to have x and y attributes, to place it on the SVG canvas. It also needs to access the data to display those values.
D3 gives you a high level of control over how you label your bars.
Example:
- The code in the editor already binds the data to each new text element. First, append text nodes to the svg. Next, add attributes for the x and y coordinates. They should be calculated the same way as the rect ones, except the y value for the text should make the label sit 3 units higher than the bar. Finally, use the D3 text() method to set the label equal to the data point value.
- Note: For the label to sit higher than the bar, decide if the y value for the text should be 3 greater or 3 less than the y value for the bar.

<body>
  <script>
    const dataset = [12, 31, 22, 17, 25, 18, 29, 14, 9];

    const w = 500;
    const h = 100;

    const svg = d3.select("body")
                  .append("svg")
                  .attr("width", w)
                  .attr("height", h);

    svg.selectAll("rect")
       .data(dataset)
       .enter()
       .append("rect")
       .attr("x", (d, i) => i * 30)
       .attr("y", (d, i) => h - 3 * d)
       .attr("width", 25)
       .attr("height", (d, i) => 3 * d)
       .attr("fill", "navy");

    svg.selectAll("text")
       .data(dataset)
       .enter()
       .append("text")
       .attr("x", (d, i) => i * 30)
       .attr("y", (d, i) => h -3 * d -3)
       .text(d => d)
  </script>
<body>

Style D3 Labels

D3 methods can add styles to the bar labels. The fill attribute sets the color of the text for a text node. The style() method sets CSS rules for other styles, such as "font-family" or "font-size".
Example: Set the font-size of the text elements to 25px, and the color of the text to red.

<body>
  <script>
    const dataset = [12, 31, 22, 17, 25, 18, 29, 14, 9];

    const w = 500;
    const h = 100;

    const svg = d3.select("body")
                  .append("svg")
                  .attr("width", w)
                  .attr("height", h);

    svg.selectAll("rect")
       .data(dataset)
       .enter()
       .append("rect")
       .attr("x", (d, i) => i * 30)
       .attr("y", (d, i) => h - 3 * d)
       .attr("width", 25)
       .attr("height", (d, i) => d * 3)
       .attr("fill", "navy");

    svg.selectAll("text")
       .data(dataset)
       .enter()
       .append("text")
       .text((d) => d)
       .attr("x", (d, i) => i * 30)
       .attr("y", (d, i) => h - (3 * d) - 3)
       .attr("font-size","25px")
       .attr("fill","red")
  </script>
</body>

Add a Hover Effect to a D3 Element

It's possible to add effects that highlight a bar when the user hovers over it with the mouse. So far, the styling for the rectangles is applied with the built-in D3 and SVG methods, but you can use CSS as well.
You set the CSS class on the SVG elements with the attr() method. Then the :hover pseudo-class for your new class holds the style rules for any hover effects.
Example: Use the attr() method to add a class of bar to all the rect elements. This changes the fill color of the bar to brown when you mouse over it.

Add a Tooltip to a D3 Element

A tooltip shows more information about an item on a page when the user hovers over that item. There are several ways to add a tooltip to a visualization, this challenge uses the SVG title element.
title pairs with the text() method to dynamically add data to the bars.
Example: Append a title element under each rect node. Then call the text() method with a callback function so the text displays the data value.

<style>
  .bar:hover {
    fill: brown;
  }
</style>
<body>
  <script>
    const dataset = [12, 31, 22, 17, 25, 18, 29, 14, 9];

    const w = 500;
    const h = 200;

    const svg = d3.select("body")
                  .append("svg")
                  .attr("width", w)
                  .attr("height", h);

    svg.selectAll("rect")
       .data(dataset)
       .enter()
       .append("rect")
       .attr("x", (d, i) => i * 30)
       .attr("y", (d, i) => h - 3 * d)
       .attr("width", 25)
       .attr("height", (d, i) => d * 3)
       .attr("fill", "navy")
       .attr("class", "bar")
       .append("title")
       .text((d) => d)
      
    svg.selectAll("text")
       .data(dataset)
       .enter()
       .append("text")
       .text((d) => d)
       .attr("x", (d, i) => i * 30)
       .attr("y", (d, i) => h - (d * 3 + 3))

  </script>
</body>

Create a Scatterplot with SVG Circles

A scatter plot is another type of visualization. It usually uses circles to map data points, which have two values each. These values tie to the x and y axes, and are used to position the circle in the visualization.
SVG has a circle tag to create the circle shape. It works a lot like the rect elements you used for the bar chart.
Example:
- Use the data(), enter(), and append() methods to bind dataset to new circle elements that are appended to the SVG canvas.
- Note: The circles won't be visible because we haven't set their attributes yet. We'll do that in the next challenge.

<body>
  <script>
    const dataset = [
                  [ 34,    78 ],
                  [ 109,   280 ],
                  [ 310,   120 ],
                  [ 79,    411 ],
                  [ 420,   220 ],
                  [ 233,   145 ],
                  [ 333,   96 ],
                  [ 222,   333 ],
                  [ 78,    320 ],
                  [ 21,    123 ]
                ];


    const w = 500;
    const h = 500;

    const svg = d3.select("body")
                  .append("svg")
                  .attr("width", w)
                  .attr("height", h);

    svg.selectAll("circle")
       .data(dataset)
       .enter()
       .append("circle")
  </script>
</body>

Add Attributes to the Circle Elements

A circle in SVG has three main attributes. The cx and cy attributes are the coordinates. They tell D3 where to position the center of the shape on the SVG canvas. The radius (r attribute) gives the size of the circle.
Just like the rect y coordinate, the cy attribute for a circle is measured from the top of the SVG canvas, not from the bottom.
All three attributes can use a callback function to set their values dynamically. Remember that all methods chained after data(dataset) run once per item in dataset. The d parameter in the callback function refers to the current item in dataset, which is an array for each point. You use bracket notation, like d[0], to access the values in that array.
Example: Add cx, cy, and r attributes to the circle elements. The cx value should be the first number in the array for each item in dataset. The cy value should be based off the second number in the array, but make sure to show the chart right-side-up and not inverted. The r value should be 5 for all circles.

<body>
  <script>
    const dataset = [
                  [ 34,    78 ],
                  [ 109,   280 ],
                  [ 310,   120 ],
                  [ 79,    411 ],
                  [ 420,   220 ],
                  [ 233,   145 ],
                  [ 333,   96 ],
                  [ 222,   333 ],
                  [ 78,    320 ],
                  [ 21,    123 ]
                ];


    const w = 500;
    const h = 500;

    const svg = d3.select("body")
                  .append("svg")
                  .attr("width", w)
                  .attr("height", h);

    svg.selectAll("circle")
       .data(dataset)
       .enter()
       .append("circle")
       .attr("cx", (d, i ) => d[0])
       .attr("cy", (d, i) => h - d[1])
       .attr("r","5")
  </script>
</body>

Add Labels to Scatter Plot Circles

Example: Label each point on the scatter plot using the text elements. The text of the label should be the two values separated by a comma and a space. For example, the label for the first point is "34, 78". Set the x attribute so it's 5 units more than the value you used for the cx attribute on the circle. Set the y attribute the same way that's used for the cy value on the circle.

<body>
  <script>
    const dataset = [
                  [ 34,    78 ],
                  [ 109,   280 ],
                  [ 310,   120 ],
                  [ 79,    411 ],
                  [ 420,   220 ],
                  [ 233,   145 ],
                  [ 333,   96 ],
                  [ 222,   333 ],
                  [ 78,    320 ],
                  [ 21,    123 ]
                ];


    const w = 500;
    const h = 500;

    const svg = d3.select("body")
                  .append("svg")
                  .attr("width", w)
                  .attr("height", h);

    svg.selectAll("circle")
       .data(dataset)
       .enter()
       .append("circle")
       .attr("cx", (d, i) => d[0])
       .attr("cy", (d, i) => h - d[1])
       .attr("r", 5);

    svg.selectAll("text")
       .data(dataset)
       .enter()
       .append("text")
       .attr("x", d => d[0] + 5)
       .attr("y", d => h- d[1])
       .text(d => `${d[0]}, ${d[1]}`)
  </script>
</body>

Create a Linear Scale with D3

The bar and scatter plot charts both plotted data directly onto the SVG canvas. However, if the height of a bar or one of the data points were larger than the SVG height or width values, it would go outside the SVG area.
In D3, there are scales to help plot data. Scales are functions that tell the program how to map a set of raw data points onto the pixels of the SVG canvas.
For example, say you have a 100x500-sized SVG canvas and you want to plot Gross Domestic Product (GDP) for a number of countries. The set of numbers would be in the billion or trillion-dollar range. You provide D3 a type of scale to tell it how to place the large GDP values into that 100x500-sized area.
It's unlikely you would plot raw data as-is. Before plotting it, you set the scale for your entire data set, so that the x and y values fit your canvas width and height.
D3 has several scale types. For a linear scale (usually used with quantitative data), there is the D3 method scaleLinear():

const scale = d3.scaleLinear()

By default, a scale uses the identity relationship. The value of the input is the same as the value of the output. A separate challenge covers how to change this.

<body>
  <script>
    const scale = d3.scaleLinear(); // Create the scale here
    const output = scale(50); // Call the scale with an argument here
      
    d3.select("body")
      .append("h2")
      .text(output);

  </script>
</body>

Set a Domain and a Range on a Scale

By default, scales use the identity relationship - the input value maps to the output value. But scales can be much more flexible and interesting.
Say a data set has values ranging from 50 to 480. This is the input information for a scale, and is also known as the domain.
You want to map those points along the x axis on the SVG canvas, between 10 units and 500 units. This is the output information, which is also known as the range.
The domain() and range() methods set these values for the scale. Both methods take an array of at least two elements as an argument. Here's an example:

// Set a domain
// The domain covers the set of input values
scale.domain([50, 480]);
// Set a range
// The range covers the set of output values
scale.range([10, 500]);
scale(50) // Returns 10
scale(480) // Returns 500
scale(325) // Returns 323.37
scale(750) // Returns 807.67
d3.scaleLinear()

Notice that the scale uses the linear relationship between the domain and range values to figure out what the output should be for a given number. The minimum value in the domain (50) maps to the minimum value (10) in the range.
Example:
- Create a scale and set its domain to [250, 500] and range to [10, 150].
- Note: You can chain the domain() and range() methods onto the scale variable.

<body>
  <script>
    const scale = d3.scaleLinear();
    scale
    .domain([250, 500])
    .range([10,150]);
      
    const output = scale(50);
    d3.select("body")
      .append("h2")
      .text(output);
  </script>
</body>

Use the d3.max and d3.min Functions to Find Minimum and Maximum Values in a Dataset

The D3 methods domain() and range() set that information for your scale based on the data. There are a couple methods to make that easier.
Often when you set the domain, you'll want to use the minimum and maximum values within the data set. Trying to find these values manually, especially in a large data set, may cause errors.
D3 has two methods - min() and max() to return this information. Here's an example:

const exampleData = [34, 234, 73, 90, 6, 52];
d3.min(exampleData) // Returns 6
d3.max(exampleData) // Returns 234

A dataset may have nested arrays, like the [x, y] coordinate pairs that were in the scatter plot example. In that case, you need to tell D3 how to calculate the maximum and minimum. Fortunately, both the min() and max() methods take a callback function. In this example, the callback function's argument d is for the current inner array. The callback needs to return the element from the inner array (the x or y value) over which you want to compute the maximum or minimum. Here's an example for how to find the min and max values with an array of arrays:

const locationData = [[1, 7],[6, 3],[8, 3]];
// Returns the smallest number out of the first elements
const minX = d3.min(locationData, (d) => d[0]);
// minX compared 1, 6, and 8 and is set to 1

Example: The positionData array holds sub arrays of x, y, and z coordinates. Use a D3 method to find the maximum value of the z coordinate (the third value) from the arrays and save it in the output variable.

<body>
  <script>
    const positionData = [[1, 7, -4],[6, 3, 8],[2, 9, 3]]
    const output = d3.max(positionData, (d) => d[2]); 

    d3.select("body")
      .append("h2")
      .text(output)
  </script>
</body>

Use Dynamic Scales

The D3 min() and max() methods are useful to help set the scale.
Given a complex data set, one priority is to set the scale so the visualization fits the SVG container's width and height. You want all the data plotted inside the SVG canvas so it's visible on the web page.
The example below sets the x-axis scale for scatter plot data. The domain() method passes information to the scale about the raw data values for the plot. The range() method gives it information about the actual space on the web page for the visualization.
In the example, the domain goes from 0 to the maximum in the set. It uses the max() method with a callback function based on the x values in the arrays. The range uses the SVG canvas' width (w), but it includes some padding, too. This puts space between the scatter plot dots and the edge of the SVG canvas.

const dataset = [
  [ 34,    78 ],
  [ 109,   280 ],
  [ 310,   120 ],
  [ 79,    411 ],
  [ 420,   220 ],
  [ 233,   145 ],
  [ 333,   96 ],
  [ 222,   333 ],
  [ 78,    320 ],
  [ 21,    123 ]
];
const w = 500;
const h = 500;

// Padding between the SVG canvas boundary and the plot
const padding = 30;
const xScale = d3.scaleLinear()
  .domain([0, d3.max(dataset, (d) => d[0])])
  .range([padding, w - padding]);

The padding may be confusing at first. Picture the x-axis as a horizontal line from 0 to 500 (the width value for the SVG canvas). Including the padding in the range() method forces the plot to start at 30 along that line (instead of 0), and end at 470 (instead of 500).
Example:
- Use the yScale variable to create a linear y-axis scale. The domain should start at zero and go to the maximum y value in the set. The range should use the SVG height (h) and include padding.
- Note: Remember to keep the plot right-side-up. When you set the range for the y coordinates, the higher value (height minus padding) is the first argument, and the lower value is the second argument.

<body>
  <script>
    const dataset = [
                  [ 34,    78 ],
                  [ 109,   280 ],
                  [ 310,   120 ],
                  [ 79,    411 ],
                  [ 420,   220 ],
                  [ 233,   145 ],
                  [ 333,   96 ],
                  [ 222,   333 ],
                  [ 78,    320 ],
                  [ 21,    123 ]
                ];

    const w = 500;
    const h = 500;

    // Padding between the SVG canvas boundary and the plot
    const padding = 30;

    // Create an x and y scale

    const xScale = d3.scaleLinear()
                    .domain([0, d3.max(dataset, (d) => d[0])])
                    .range([padding, w - padding]);

    const yScale = d3.scaleLinear()
                     .domain([0,d3.max(dataset, (d) => d[1])])
                     .range([h - padding, padding])

    const output = yScale(411); // Returns 30
    d3.select("body")
      .append("h2")
      .text(output)
  </script>
</body>

Use a Pre-Defined Scale to Place Elements

With the scales set up, it's time to map the scatter plot again. The scales are like processing functions that turn the x and y raw data into values that fit and render correctly on the SVG canvas. They keep the data within the screen's plotting area.
You set the coordinate attribute values for an SVG shape with the scaling function. This includes x and y attributes for rect or text elements, or cx and cy for circles. Here's an example:

shape
  .attr("x", (d) => xScale(d[0]))

Scales set shape coordinate attributes to place the data points onto the SVG canvas. You don't need to apply scales when you display the actual data value, for example, in the text() method for a tooltip or label.
Example:
- Use xScale and yScale to position both the circle and text shapes onto the SVG canvas. For the circles, apply the scales to set the cx and cy attributes. Give them a radius of 5 units, too.
- For the text elements, apply the scales to set the x and y attributes. The labels should be offset to the right of the dots. To do this, add 10 units to the x data value before passing it to the xScale.

<body>
  <script>
    const dataset = [
                  [ 34,     78 ],
                  [ 109,   280 ],
                  [ 310,   120 ],
                  [ 79,   411 ],
                  [ 420,   220 ],
                  [ 233,   145 ],
                  [ 333,   96 ],
                  [ 222,    333 ],
                  [ 78,    320 ],
                  [ 21,   123 ]
                ];

    const w = 500;
    const h = 500;
    const padding = 60;

    const xScale = d3.scaleLinear()
                     .domain([0, d3.max(dataset, (d) => d[0])])
                     .range([padding, w - padding]);

    const yScale = d3.scaleLinear()
                     .domain([0, d3.max(dataset, (d) => d[1])])
                     .range([h - padding, padding]);

    const svg = d3.select("body")
                  .append("svg")
                  .attr("width", w)
                  .attr("height", h);

    svg.selectAll("circle")
       .data(dataset)
       .enter()
       .append("circle")
       .attr("cx", (d) => xScale(d[0]))
       .attr("cy", (d) => yScale(d[1]))
       .attr("r",5);

    svg.selectAll("text")
       .data(dataset)
       .enter()
       .append("text")
       .text((d) =>  (d[0] + ", " + d[1]))
       .attr("x", (d) => xScale(d[0]+10))
       .attr("y", (d) => yScale(d[1]))

  </script>
</body>

Add Axes to a Visualization

D3 has two methods axisLeft() and axisBottom() to render the y and x axes, respectively. (Axes is the plural form of axis). Here's an example to create the x-axis based on the xScale in the previous challenges:

const xAxis = d3.axisBottom(xScale);

The next step is to render the axis on the SVG canvas. To do so, you can use a general SVG component, the g element. The g stands for group.
Unlike rect, circle, and text, an axis is just a straight line when it's rendered. Because it is a simple shape, using g works.
The last step is to apply a transform attribute to position the axis on the SVG canvas in the right place. Otherwise, the line would render along the border of SVG canvas and wouldn't be visible.
SVG supports different types of transforms, but positioning an axis needs translate. When it's applied to the g element, it moves the whole group over and down by the given amounts. Here's an example:

const xAxis = d3.axisBottom(xScale);

svg.append("g")
   .attr("transform", "translate(0, " + (h - padding) + ")")
   .call(xAxis);

The above code places the x-axis at the bottom of the SVG canvas. Then it's passed as an argument to the call() method. The y-axis works in the same way, except the translate argument is in the form (x, 0). Because translate is a string in the attr() method above, you can use concatenation to include variable values for its arguments.
Example: The scatter plot now has an x-axis. Create a y-axis in a variable named yAxis using the axisLeft() method. Then render the axis using a g element. Make sure to use a transform attribute to translate the axis by the amount of padding units right, and 0 units down. Remember to call() the axis.

<body>
  <script>
    const dataset = [
                  [ 34,     78 ],
                  [ 109,   280 ],
                  [ 310,   120 ],
                  [ 79,   411 ],
                  [ 420,   220 ],
                  [ 233,   145 ],
                  [ 333,   96 ],
                  [ 222,    333 ],
                  [ 78,    320 ],
                  [ 21,   123 ]
                ];

    const w = 500;
    const h = 500;
    const padding = 60;

    const xScale = d3.scaleLinear()
                     .domain([0, d3.max(dataset, (d) => d[0])])
                     .range([padding, w - padding]);

    const yScale = d3.scaleLinear()
                     .domain([0, d3.max(dataset, (d) => d[1])])
                     .range([h - padding, padding]);

    const svg = d3.select("body")
                  .append("svg")
                  .attr("width", w)
                  .attr("height", h);

    svg.selectAll("circle")
       .data(dataset)
       .enter()
       .append("circle")
       .attr("cx", (d) => xScale(d[0]))
       .attr("cy",(d) => yScale(d[1]))
       .attr("r", (d) => 5);

    svg.selectAll("text")
       .data(dataset)
       .enter()
       .append("text")
       .text((d) =>  (d[0] + "," + d[1]))
       .attr("x", (d) => xScale(d[0] + 10))
       .attr("y", (d) => yScale(d[1]))

    const xAxis = d3.axisBottom(xScale);
    const yAxis = d3.axisLeft(yScale);

    svg.append("g")
       .attr("transform", "translate(0," + (h - padding) + ")")
       .call(xAxis);

    svg.append("g")
       .attr("transform", "translate(" + padding +", 0)")
       .call(yAxis)

  </script>
</body>

akirap3/01_Data_Visualization_DataVisualizationWithD3.md

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Introduction to the Data Visualization with D3

Add Document Elements with D3

Select a Group of Elements with D3

Work with Data in D3

Work with Dynamic Data in D3

Soulution 1:

Solution 2:

Add Inline Styling to Elements

Change Styles Based on Data

Add Classes with D3

Update the Height of an Element Dynamically

Change the Presentation of a Bar Chart

Learn About SVG in D3

Display Shapes with SVG

Create a Bar for Each Data Point in the Set

Dynamically Set the Coordinates for Each Bar

Dynamically Change the Height of Each Bar

Invert SVG Elements

Change the Color of an SVG Element

Add Labels to D3 Elements

Style D3 Labels

Add a Hover Effect to a D3 Element

Add a Tooltip to a D3 Element

Create a Scatterplot with SVG Circles

Add Attributes to the Circle Elements

Add Labels to Scatter Plot Circles

Create a Linear Scale with D3

Set a Domain and a Range on a Scale

Use the d3.max and d3.min Functions to Find Minimum and Maximum Values in a Dataset

Use Dynamic Scales

Use a Pre-Defined Scale to Place Elements

Add Axes to a Visualization