D3 Examples - Simple vertical d3.js tree diagram

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<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8">
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    <title>Collapsible Tree Example</title>
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    <style>
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    .node circle {
      fill: #fff;
      stroke: red;
      stroke-width: 2px;
    }
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    .node text { font: 12px sans-serif; }
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    .link {
      fill: none;
      stroke: #ccc;
      stroke-width: 2px;
    }
    
    </style>
​
  </head>
​
  <body>
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<!-- load the d3.js library --> 
<script src="https://d3js.org/d3.v3.min.js"></script>
    
<script>
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var treeData = [
  {
    "name": "United States of America",
    "parent": "null",
    "children": [
      {
        "name": "States",
        "parent": "United States of America",
        "children": [
          {
            "name": "Counties",
            "parent": "States"
            
          },
          {
            "name": "Places",
            "parent": "States"
          }
        ]
      },
      {
        "name": "Zip Codes (ZCTA)",
        "parent": "United States of America"
      },
      {
        "name": "114th CD",
        "parent": "United States of America"
      }
    ]
  }
];
​
// ************** Generate the tree diagram  *****************
var margin = {top: 40, right: 20, bottom: 20, left: 20},
    width = 500 - margin.right - margin.left,
    height = 600 - margin.top - margin.bottom;
    
var i = 0;
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var tree = d3.layout.tree()
    .size([height, width]);
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var diagonal = d3.svg.diagonal()
    .projection(function(d) { return [d.x, d.y]; });
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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) {
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  // 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 * 100; });
​
  // Declare the nodes…
  var node = svg.selectAll("g.node")
      .data(nodes, function(d) { return d.id || (d.id = ++i); });
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  // Enter the nodes.
  var nodeEnter = node.enter().append("g")
      .attr("class", "node")
      .attr("transform", function(d) { 
          return "translate(" + d.x + "," + d.y + ")"; });
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  nodeEnter.append("circle")
      .attr("r", 5)
      .style("fill", "#fff");
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  nodeEnter.append("text")
      .attr("y", function(d) { 
          return d.children || d._children ? -20 : 20; })
      .attr("dy", ".35em")
      .attr("text-anchor", "middle")
      .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; });
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  // Enter the links.
  link.enter().insert("path", "g")
      .attr("class", "link")
      .attr("d", diagonal);
​
}
​
</script>
    
  </body>
</html>
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