Force Directed Graph D3 JS - National Contiguity

app.js

var url = "https://raw.githubusercontent.com/DealPete/forceDirected/master/countries.json";

d3.json(url, function(json){
  
  var data = json;
  
  //Set dimensions of div container, SVG and chart area(g element)
  var margin = {top: 40, right: 40, bottom: 40, left: 40};
  
  //Width of the visualization area
  var w = 1000 - margin.left - margin.right;
  var h = 1000 - margin.top - margin.bottom;
  
  //Create SVG element and append to #chart div container
  var svg = d3.select("#chart")
              .append("svg")
                .attr("width", w + margin.left + margin.right)
                .attr("height", h + margin.top + margin.bottom)
              .append("g")
                .attr("transform", "translate(" + margin.left + "," + margin.top + ")")
                .attr("id", "area");
  
  //Extract nodes and links data
  var nodes = data.nodes;
  var links = data.links;
  
  //Get position of g element of the chart
  var position = document.getElementById("area").getBoundingClientRect();
  
    
  //Append tooltip to chart area. Fully transparant at first. Use mouse events to show: making less opaque
  var tip = d3.select("#chart").append("div")
                               .attr("class", "tooltip")
                               .style("opacity", 0);
  

  //Create a force layout object and define its properties
  var force = d3.layout.force()
                .size([w,h])
                .nodes(nodes)
                .links(links);
  
  force.linkDistance(h/20);
  //desired distance between connected nodes
  //charge influences the linkDistance
  
  force.charge(-120)
  //charge is like electrical charge causing the nodes to repel eachother
  //to prevent nodes from overlapping eachother
  
  //force.linkStrength(0.5);
  //linkStrength influences the linkDistance
  //how strong to maintain the linkDistance
  
  //force.gravity(0.2);
  //how much the whole visualization is attracted to the edges
  
  //append a link to SVG container for every link data object (no position yet)
  var link = svg.selectAll(".link")
                .data(links)
                .enter()
                .append("line")
                .attr("class", "link");
  
  //append a node(circle element) to SVG container for every node data object(no position yet)
  var node = d3.select("#chart").selectAll(".node")
                .data(nodes)
                .enter()
                .append("div")
                .call(force.drag);
  
  
  force.on("tick", function(){
  //set node and link position attributes once force calculations have finished  

    //position the nodes
    //force adds a x and y property to each node object
    node.style("left", function(d){
      
            return d.x  + position.left + "px";
            
        })
        .style("top", function(d){
      
            return d.y  + position.top  +  "px";
      
        })
        .attr("class", function(d){
      
            return "flag flag-" + d.code + " node";
      
        })
        .attr("src", "https://res.cloudinary.com/dettjqo9j/image/upload/v1485942660/flags_xf9dde.png")
        .on("mouseover", function(d){
        //show tooltip on mouseover
            
            tip.transition()
               .style("opacity", 0.7);
      
            tip.html("<strong>" + d.country + "</strong>")
               .style("left", d3.event.pageX + "px")
               .style("top", d3.event.pageY - 50 + "px");
      
        })
        .on("mouseout", function(d){
      
            tip.transition()
               .style("opacity", 0);
      
        });
    
    link.attr("x1", function(d){
    //force adds an x and y property to each source and target object of link array
      
            return d.source.x;
      
        })
        .attr("y1", function(d){
      
            return d.source.y;
      
        })
        .attr("x2", function(d){
      
            return d.target.x;
      
        })
        .attr("y2", function(d){
      
             return d.target.y;
      
        })
 
  })//force.on
                              
  force.start();
  //let the force start its calculations

});//d3.json

index.html

<head>
  <meta charset="utf-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link href="https://fonts.googleapis.com/css?family=Fira+Sans" rel="stylesheet">
</head>

<body>
  <div class="container text-center">
    <h1>Which Countries Share Borders?</h1>
    <h2>A Force-Directed Graph with D3 JS</h2>
    <br>
    <div id="chart">
    </div>
  </div>
</body>

style.css

body {
  margin-top: 20px;
  font-family: 'Fira Sans', sans-serif;
  background-image: url("https://subtlepatterns.com/patterns/sos.png");
}

svg {
  background-color: white;
  box-shadow: 0 0 10px #888888;
}

.node {
  position: absolute;
}

.link {
  stroke: #2c3e50;
  stroke-width: 2px;
}

.tooltip {
  position: absolute;
  text-align: center;
  vertical-align: middle;
  width: 140px;
  height: 50px;
  padding: 10px;
  font: 14px Fira Sans;
  background: #2c3e50;
  border: 0px;
  border-radius: 8px;
  color: white;
}


/*!
 * Generated with CSS Flag Sprite generator (https://www.flag-sprites.com/)
 */

.flag {
  display: inline-block;
  position: absolute;
  width: 16px;
  height: 11px;
  background: url('https://res.cloudinary.com/dettjqo9j/image/upload/v1485942660/flags_xf9dde.png') no-repeat;
}

.flag.flag-ml {
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}

.flag.flag-ch {
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.flag.flag-gw {
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.flag.flag-qa {
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  background-position: -112px 0;
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.flag.flag-pn {
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  background-position: -16px -22px;
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.flag.flag-bh {
  background-position: -80px -11px;
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.flag.flag-ge {
  background-position: -176px -44px;
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  background-position: -208px -110px;
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.flag.flag-us {
  background-position: -128px -154px;
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.flag.flag-mu {
  background-position: -112px -99px;
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.flag.flag-eh {
  background-position: -192px -33px;
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.flag.flag-ar {
  background-position: -144px 0;
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.flag.flag-bv {
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.flag.flag-ec {
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.flag.flag-cn {
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.flag.flag-pk {
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.flag.flag-dj {
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.flag.flag-ke {
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.flag.flag-pa {
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.flag.flag-pr {
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.flag.flag-uy {
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}

.flag.flag-za {
  background-position: -128px -165px;
}

.flag.flag-sc {
  background-position: 0 -132px;
}

.flag.flag-li {
  background-position: -240px -77px;
}

.flag.flag-ie {
  background-position: -48px -66px;
}

.flag.flag-pw {
  background-position: -96px -121px;
}

.flag.flag-gf {
  background-position: -192px -44px;
}

.flag.flag-vi {
  background-position: -240px -154px;
}