Tree diagram

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<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8">

<title>Collapsible Tree Example</title>

<style>

.node circle {
	  fill: #fff;
	  stroke: steelblue;
	  stroke-width: 3px;
	}

.node text { font: 12px sans-serif; }

.link {
	  fill: none;
	  stroke: #ccc;
	  stroke-width: 2px;
	}
	
    </style>

</head>

<body>

<script src="https://d3js.org/d3.v3.min.js"></script>
	
<script>

var data = [
    { "name" : "4", "child":"null" },
    { "name" : "3", "child":"4" },
    { "name" : "2", "child":"4" },
    { "name" : "1", "child":"3" },
    { "name" : "2", "child":"3" }
    ];

// *********** Convert flat data into a nice tree ***************
// create a name: node map
var dataMap = data.reduce(function(map, node) {
	map[node.name] = node;
	return map;
}, {});

// create the tree array
var treeData = [];
data.forEach(function(node) {
	// add to parent
	var child = dataMap[node.child];
	if (child) {
		// create child array if it doesn't exist
		(child.children || (child.children = []))
			// add node to child array
			.push(node);
	} else {
		// parent is null or missing
		treeData.push(node);
	}
});

// ************** Generate the tree diagram	 *****************
var margin = {top: 360, right: 20, bottom: 20, left: 120},
	width = 960 - margin.right - margin.left,
	height = 500 - margin.top - margin.bottom;
	
var i = 0;

var tree = d3.layout.tree()
	.size([height, width]);

var diagonal = d3.svg.diagonal()
	.projection(function(d) { return [d.x, d.y]; });

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

root = treeData[0];
  
update(root);

function update(source) {

// Compute the new tree layout.
  var nodes = tree.nodes(root).reverse(),
	  links = tree.links(nodes);

// Normalize for fixed-depth.
  nodes.forEach(function(d) { d.y = -d.depth * 80; });

// Declare the nodes…
  var node = svg.selectAll("g.node")
	  .data(nodes, function(d) { return d.id || (d.id = ++i); });

// Enter the nodes.
  var nodeEnter = node.enter().append("g")
	  .attr("class", "node")
	  .attr("transform", function(d) { 
		  return "translate(" + d.x + "," + d.y + ")"; });

nodeEnter.append("circle")
	  .attr("r", 10)
	  .style("fill", "#fff");

nodeEnter.append("text")
	  .attr("x", function(d) { 
		  return d.children || d._children ? -18 : 18; })
	  .attr("dy", ".35em")
	  .attr("text-anchor", function(d) { 
		  return d.children || d._children ? "end" : "start"; })
	  .text(function(d) { return d.name; })
	  .style("fill-opacity", 1);

// Declare the links…
  var link = svg.selectAll("path.link")
	  .data(links, function(d) { return d.target.id; });

// Enter the links.
  link.enter().insert("path", "g")
	  .attr("class", "link")
	  .attr("d", diagonal);

}

</script>
	
  </body>
</html>

Modified http://d3js.org/d3.v3.min.js to a secure url

https://d3js.org/d3.v3.min.js