title: Node size proportional to number of children in D3
body: <p>I created this click-expand-collapse network -<a href="http://jsfiddle.net/5Lv8gkqv/" rel="nofollow">http://jsfiddle.net/5Lv8gkqv/</a></p>
  
  <pre><code>var width = 960,
      height = 500,
      root = {
   "name": "Chocolate", "tag":"class",
   "children": [
    {
     "name": "Wafer", "tag":"subclass",
     "children": [
      {
        "name": "Nestle", "tag":"company",
       "children": [
        {"name": "KitKat", "tag":"product"}
       ]
      }
     ]
    },
  
    {
     "name": "White", "tag":"subclass",
     "children": [
      {
        "name": "Nestle", "tag":"company",
       "children": [
        {"name": "Milkybar", "tag":"product"}
       ]
      }
     ]
    },
  
      {
     "name": "Caramel", "tag":"subclass",
     "children": [
      {
        "name": "Nestle", "tag":"company",
       "children": [
        {"name": "BarOne", "tag":"product"}
       ]
      }
     ]
    },    
      {
     "name": "Milk", "tag":"subclass",
     "children": [
      {
        "name": "Nestle", "tag":"company",
       "children": [
        {"name": "Nestle Milk", "tag":"product"}
       ]
      },  {
        "name": "Cadbury", "tag":"company",
       "children": [
        {"name": "Dairy Milk", "tag":"product"}
       ]
      }
     ]
    }
  
  
  
  
   ]
  };
  
  var force = d3.layout.force()
      .linkDistance(150)
      .charge(-120)
      .gravity(.05)
      .size([width, height])
      .on("tick", tick);
  
  var svg = d3.select("body").append("svg")
      .attr("width", width)
      .attr("height", height);
  
  var link = svg.selectAll(".link"),
      node = svg.selectAll(".node");
  
  flatten(root); //to set ids
  setParents(root, null);
  collapseAll(root);
  root.children = root._children;
  root._children = null;
  update();
  
  function update() {
    var nodes = flatten(root),
        links = d3.layout.tree().links(nodes);
    // Restart the force layout.
    force
        .nodes(nodes)
        .links(links)
        .start();
  
    // Update links.
    link = link.data(links, function(d) { return d.target.id; });
  
    link.exit().remove();
  
    link.enter().insert("line", ".node")
        .attr("class", "link");
  
    // Update nodes.
    node = node.data(nodes, function(d) { return d.id; });
  
    node.exit().remove();
  
    var nodeEnter = node.enter().append("g")
        .attr("class", "node")
        .on("click", click)
        .call(force.drag);
  
    nodeEnter.append("circle")
        .attr("r", function(d) { return Math.sqrt(d.size) / 10 || 4.5; });
  
    nodeEnter.append("text")
        .attr("dy", ".35em")
        .text(function(d) { return d.name; });
  
    node.select("circle")
        .style("fill", color);
  }
  
  function tick() {
    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("transform", function(d) { return "translate(" + d.x + "," + d.y + ")"; });
  }
  
  function color(d) {
    return d._children ? "#3182bd" // collapsed package
        : d.children ? "#c6dbef" // expanded package
        : "#fd8d3c"; // leaf node
  }
  
  // Toggle children on click.
  function click(d) {
    if (d3.event.defaultPrevented) return; // ignore drag
    if (d.children) {
        collapseAll(d);
    } else {
        if (d._parent){
            d._parent.children.forEach(function(e){
                if (e != d){
                    collapseAll(e);
                }
            });
        }
      d.children = d._children;
      d._children = null;
    }
    update();
  }
  
  function collapseAll(d){
      if (d.children){
          d.children.forEach(collapseAll);
          d._children = d.children;
          d.children = null;
      }
      else if (d._childred){
          d._children.forEach(collapseAll);
      }
  }
  
  // Returns a list of all nodes under the root.
  function flatten(root) {
    var nodes = [], i = 0;
  
    function recurse(node) {
      if (node.children) node.children.forEach(recurse);
      if (!node.id) node.id = ++i;
      nodes.push(node);
    }
    recurse(root);
    return nodes;
  }
  
  function setParents(d, p){
      d._parent = p;
    if (d.children) {
        d.children.forEach(function(e){ setParents(e,d);});
    } else if (d._children) {
        d._children.forEach(function(e){ setParents(e,d);});
    }
  }
  </code></pre>
  
  <p>The thing now is that I was wondering if its possible to have node-size proportional to the number of children. So the parent node will be the biggest circle and the leaf node will be the smallest, while the intermediate node-size will be dependent on the number of children each have.</p>