29.Force-Directed Graph

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<!DOCTYPE html>
<meta charset="utf-8">
<style>

.links line {
  stroke: #999;
  stroke-opacity: 0.6;
}

.nodes circle {
  stroke: #fff;
  stroke-width: 1.5px;
}

</style>

var svg = d3.select("svg"),
    width = +svg.attr("width"),
    height = +svg.attr("height");

var color = d3.scaleOrdinal(d3.schemeCategory20);

var simulation = d3.forceSimulation()
    .force("link", d3.forceLink().id(function(d) { return d.id; }))
    .force("charge", d3.forceManyBody())
    .force("center", d3.forceCenter(width / 2, height / 2));

d3.json("miserables.json", function(error, graph) {
  if (error) throw error;

// console.log(graph.links);  
  
  var link = svg.append("g")
      .attr("class", "links")
    .selectAll("line")
    .data(graph.links)
    .enter().append("line")
      .attr("stroke-width", function(d) { return Math.sqrt(d.value); });

var node = svg.append("g")
      .attr("class", "nodes")
    .selectAll("circle")
    .data(graph.nodes)
    .enter().append("circle")
      .attr("r", 5)
      .attr("fill", function(d) { return color(d.group); })
//       .call(d3.drag()
//           .on("start", dragstarted)
//           .on("drag", dragged)
//           .on("end", dragended));

node.append("title")
      .text(function(d) { return d.id; });

// ---- by now, node's data is the same as original data.node in data file
  console.log(node);
	console.log(link);

// --- apply graph.nodes data to simulation.nodes()
  simulation
      .nodes(graph.nodes)
//       .on("tick", ticked);
  
   
// --- apply graph.links data to simulation.links()
  simulation.force("link")
      .links(graph.links);
// then, console.log(link) has more data elements than original data.link
  
   
// ---- after 4 lines code above, node's data has more than original graph.node/.link in data file
  console.log(node);
	console.log(link);   
//   node and link has x,y, vx, vy, but not yet applied to svg elements

// ---- following ticked() and simulation.on("tick") apply x,y,vx,vy to svg elements to display node and link
  function ticked() {
    link
        .attr("x1", function(d) { 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; });

node
        .attr("cx", function(d) { return d.x; })
        .attr("cy", function(d) { return d.y; });
  }
  
  simulation.on("tick", ticked);
  
// ---- by now, we can drag and move 
  node
        .call(d3.drag()
          .on("start", dragstarted)
          .on("drag", dragged)
          .on("end", dragended));
  
});

function dragstarted(d) {
  if (!d3.event.active) simulation.alphaTarget(0.3).restart();
  d.fx = d.x;
  d.fy = d.y;
}

function dragged(d) {
  d.fx = d3.event.x;
  d.fy = d3.event.y;
}

function dragended(d) {
  if (!d3.event.active) simulation.alphaTarget(0);
  d.fx = null;
  d.fy = null;
}

</script>

https://d3js.org/d3.v4.min.js