Bottom to Top Force Tree layout

<!DOCTYPE html>
<html>
  <head>
    <title>Force-Directed Layout</title>
    <script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/d3/3.4.13/d3.min.js"></script>
    <style type="text/css">

circle {
  stroke-width: 1.5px;
}

line {
  stroke: #999;
}

</style>
  </head>
  <body>
    <script type="text/javascript">

var w = 960,
    h = 500,
    r = 6,
    fill = d3.scale.category20();

var force = d3.layout.force()
    .charge(-120)
    .linkDistance(30)
    .size([w, h]);

var svg = d3.select("body").append("svg:svg")
    .attr("width", w)
    .attr("height", h);

d3.json("graph.json", function(json) {
  var link = svg.selectAll("line")
      .data(json.links)
    .enter().append("svg:line");

var node = svg.selectAll("circle")
      .data(json.nodes)
    .enter().append("svg:circle")
      .attr("r", r - .75)
      .style("fill", function(d) { return fill(d.group); })
      .style("stroke", function(d) { return d3.rgb(fill(d.group)).darker(); })
      .call(force.drag);

force
      .nodes(json.nodes)
      .links(json.links)
      .on("tick", tick)
      .start();

function tick(e) {

// Push sources up and targets down to form a weak tree.
    var k = 6 * e.alpha;
    json.links.forEach(function(d, i) {
      d.source.y += k;
      d.target.y -= k;
    });

node.attr("cx", function(d) { return d.x; })
        .attr("cy", function(d) { return d.y; });

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; });
  }
});

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

https://cdnjs.cloudflare.com/ajax/libs/d3/3.4.13/d3.min.js