bar chart editor step 1

index.js

// all of these variables are independent of the data,
// so we exclude them from the draw function
var margin = {top: 20, right: 30, bottom: 30, left: 40},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

var x = d3.scale.ordinal()
    .rangeRoundBands([0, width], .1);

var y = d3.scale.linear()
    .range([height, 0]);

var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom");

var yAxis = d3.svg.axis()
    .scale(y)
    .orient("left");

var chart = d3.select(".chart")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

var gXAxis = chart.append("g")
    .attr("class", "x axis")
    .attr("transform", "translate(0," + height + ")");

var gYAxis = chart.append("g")
    .attr("class", "y axis");

// this is the function that will be called each time we draw the chart
function draw () {
  d3.tsv("data.tsv", type, function(error, data) {
    // the data may have changed so we need to update the domain of our scales
    x.domain(data.map(function(d) { return d.name; }));
    y.domain([0, d3.max(data, function(d) { return d.value; })]);

    // we update the axis to reflect the new scale domains
    gXAxis.call(xAxis);
    gYAxis.call(yAxis);

    var bars = chart.selectAll(".bar").data(data);

    // if there are new bars since we last called draw (or if it's the first time),
    // we add them now
    bars
      .enter().append("rect")
        .attr("class", "bar");

    // we update the bars with new attributes based on the new data
    bars
        .attr("x", function(d) { return x(d.name); })
        .attr("y", function(d) { return y(d.value); })
        .attr("height", function(d) { return height - y(d.value); })
        .attr("width", x.rangeBand());

    // maybe some bars are not present in the new data, let's remove them!
    bars.exit().remove();
  });
}

function type(d) {
  d.value = +d.value; // coerce to number
  return d;
}

draw();
// we can call draw any number of time, axis won't be re drawn, just updated
draw();
// bars won't superpose, they'll just be updated or removed
draw();
// such is the magic of d3 data joins! http://bost.ocks.org/mike/join/
draw();