text: var r = 960,
      format = d3.format(",d"),
      fill = d3.scale.category20c();
  
  var bubble = d3.layout.pack()
      .sort(null)
      .size([r, r])
      .padding(1.5);
  
  var vis = d3.select("#chart").append("svg")
      .attr("width", r)
      .attr("height", r)
      .attr("class", "bubble");
  
  d3.json("../data/flare.json", function(json) {
    var node = vis.selectAll("g.node")
        .data(bubble.nodes(classes(json))
        .filter(function(d) { return !d.children; }))
      .enter().append("g")
        .attr("class", "node")
        .attr("transform", function(d) { return "translate(" + d.x + "," + d.y + ")"; });
  
    node.append("title")
        .text(function(d) { return d.className + ": " + format(d.value); });
  
    node.append("circle")
        .attr("r", function(d) { return d.r; })
        .style("fill", function(d) { return fill(d.packageName); });
  
    node.append("text")
        .attr("text-anchor", "middle")
        .attr("dy", ".3em")
        .text(function(d) { return d.className.substring(0, d.r / 3); });
  });
  
  // Returns a flattened hierarchy containing all leaf nodes under the root.
  function classes(root) {
    var classes = [];
  
    function recurse(name, node) {
      if (node.children) node.children.forEach(function(child) { recurse(node.name, child); });
      else classes.push({packageName: name, className: node.name, value: node.size});
    }
  
    recurse(null, root);
    return {children: classes};
  }
  