text: var margin = {top: 50, right: 120, bottom: 20, left: 120},
    width = 960 - margin.right - margin.left,
    height = 500 - margin.top - margin.bottom;
    
  var i = 0,
      duration = 750,
      root;
  
  var tree = d3.layout.tree()
    .size([height, width]);
  
  // declare variables for drawing links between nodes
  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 + ")");
  
  // var force = d3.layout.force()
  //   .gravity(.05)
  //   .distance(100)
  //   .charge(-100)
  //   .size([width, height]);
  
  // load the external data
  d3.json("treeData.json", function(error, treeData, json) {
    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 * 100; });
    // Declare the nodes…
    var node = svg.selectAll("g.node")
      .data(nodes, function(d) { return d.id || (d.id = ++i); });
    // Enter the nodes to determined position
    var nodeEnter = node.enter().append("g")
      .attr("class", "node")
      .attr("transform", function(d) { 
        return "translate(" + d.x + "," + d.y + ")"; });
    nodeEnter.append("circle")
      .attr("r", 8)
      .style("fill", "#fff");
    nodeEnter.append("text")
      .attr("y", function(d) { 
        return d.children || d._children ? -18 : 18; })
      .attr("dy", ".35em")
      .attr("text-anchor", "middle")
      .text(function(d) { return d.name; })
      .style("fill-opacity", 1);
  
    // Transition nodes on update
    var nodeUpdate = node.transition()
      .duration(duration)
      .attr("transform", function(d) { return "translate(" + d.x + "," + d.y + ")"; });
    nodeUpdate.select("circle")
      .attr("r", 10)
      .style("fill", function(d) { return d._children ? "lightsteelblue" : "#fff"; });
    nodeUpdate.select("text")
      .style("fill-opacity", 1);
    // Transition exiting nodes to the parent's new position.
    var nodeExit = node.exit().transition()
      .duration(duration)
      .attr("transform", function(d) { return "translate(" + source.x + "," + source.y + ")"; })
      .remove();
    nodeExit.select("circle")
      .attr("r", 1e-6);
    nodeExit.select("text")
      .style("fill-opacity", 1e-6);
  
    // 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", function(d) {
        var o = {x: source.x0, y: source.y0};
        return diagonal({source: o, target: o});
      });
      // .attr("d", diagonal);
  
    // Transition links to their new position.
    link.transition()
      .duration(duration)
      .attr("d", diagonal);
    // Transition exiting nodes to the parent's new position.
    link.exit().transition()
      .duration(duration)
      .attr("d", function(d) {
        var o = {x: source.x, y: source.y};
        return diagonal({source: o, target: o});
      })
      .remove();
    // Stash the old positions for transition.
    nodes.forEach(function(d) {
      d.x0 = d.x;
      d.y0 = d.y;
    });
  }
  
  // add interaction
  function click(d) {
    if (d.children) {
      d._children = d.children;
      d.children = null;
    } else {
      d.children = d._children;
      d._children = null;
    }
    update(d);
  }