D3 Examples - Force layout

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<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
  </style>
</head>
​
<body>
  <script>
    // Feel free to change or delete any of the code you see in this editor!
var margin = {top:20, bottom:20, left:20, right:20};
var width = 600 - margin.left - margin.right;
var height = 400 - margin.top - margin.bottom;
var radius = 8;
var colors = d3.scaleOrdinal(d3.schemeCategory10);  
    
var svg = d3.select("body").append("svg")
      .attr("width", width)
      .attr("height", height)
​
d3.json('miserables.json', function(err,data){
  data.nodes.forEach(function(node){
    var perRow = 4
    node.focusX = (node.group % perRow) * 40;
    node.focusY = Math.floor(node.group/perRow)*40;
  });
  var drag = d3.drag()
        .on('start', function(){
        if (!d3.event.active){
          simulation.alphaTarget(0.35).restart();
        }
      })
        .on('end', function(){
        simulation.alphaTarget(0);
        d3.event.subject.fx=null;
            d3.event.subject.fy=null;
      })
        
        .on('drag', function(){
//       console.log(d3.event);
      d3.event.subject.fx = d3.event.x;
      d3.event.subject.fy = d3.event.y;
    });
  
  var simulation = d3.forceSimulation(data.nodes)
        .force('center', d3.forceCenter(width/2, height/2)) 
        .force('collide', d3.forceCollide(radius+15))
    .force('y', d3.forceY().y(function(d){
    return d.focusY
  }))
  .force('links', d3.forceLink(data.links).id(function(d){return d.id}))
  .on('tick', ticked);
​
 var links = svg.selectAll("line")
        .data(data.links)
        .enter().append('line')
        .attr('stroke','#ccc');
 var circles = svg.selectAll("circle")
    .data(data.nodes, function(d){return d.id})
    .enter().append("circle")
    .attr("r", radius)
        .attr('fill', function(d){return colors(d.group)})
        .call(drag);
​
  function ticked(){
    circles.attr('cx', function(d) {return d.x})
            .attr('cy', function (d) {return d.y})
    links.attr('x1', function(d){return d.source.x})
            .attr('x2', function(d) {return d.target.x})
            .attr('y1', function(d) {return d.source.y})
            .attr('y2', function(d) {return d.target.y})
    
//     console.log('tick', data.nodes[60].x);
  }
});
​
  </script>
</body>
​
    