D3 Examples - D3 Force Simulation

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<!DOCTYPE html>
<head>
  <meta charset="utf-8">
​
  <style>
  </style>
</head>
​
<body>
  <h1>Force Directed layout</h1>
  <svg 
    width=400 
    height=400
    id="network"></svg>
  
    <script src="https://d3js.org/d3.v4.min.js"></script>
  <script>
    // Copy drag from: https://bl.ocks.org/mbostock/4062045
    
    let svg = d3.select("#network"),
        width = +svg.attr("width"),
        height = + svg.attr("height"),
        N = 100;
    
    let nodes = d3.range(N).map(() => {
      return {
//         x: Math.random()*width,
//         y: Math.random()*height
      };              
    });
    
    let links = d3.range(N*3).map(() => {
      return {
        source:Math.floor(Math.random()*N),
        target:Math.floor(Math.random()*N)
      }
    });
    console.log(width, height);
    
    let simulation = d3.forceSimulation(nodes)
        .force("center", d3.forceCenter(width/2, height/2))
        .force("charge", d3.forceManyBody().strength(-5))
        .force("link", d3.forceLink(links)
                .distance(50))
    
        .on("tick", ticked);
        
    console.log("nodes", nodes);
    console.log("links", links);    
    
    let sellinks = svg.selectAll(".link")
        .data(links)
        .enter()
        .append("line")
        .attr("class", "link")
      .style("stroke", "#aaa")
      .style("opacity", 0.9)
    
    let selnodes = svg.selectAll(".node")
        .data(nodes)
        .enter()
        .append("circle")
        .attr("class", "node")
      .style("fill", "steelblue")
      .attr("r", 3.5)
​
   function ticked() {
            sellinks.attr("x1", (l) => l.source.x)
        .attr("y1", (l) => l.source.y)
        .attr("x2", (l) => l.target.x)
        .attr("y2", (l) => l.target.y);
​
      selnodes.attr("cx", (d) => d.x)
          .attr("cy", (d) => d.y);
     
//      console.log("ticked!");
​
   }
    
​
  </script>
</body>
​