index.html

## index.html
<!DOCTYPE html>
<html>
  <head>
    <meta http-equiv="Content-Type" content="text/html;charset=utf-8"/>
    <title>Beeswarm plot - collision detection</title>
    <script type="text/javascript" src="http://mbostock.github.com/d3/d3.js"></script>
    <script type="text/javascript" src="http://mbostock.github.com/d3/d3.geom.js"></script>
    <script type="text/javascript" src="http://mbostock.github.com/d3/d3.layout.js"></script>
  </head>
  <body>
    <div id="body">
      <div id="chart"></div>
    </div>
    <script type="text/javascript">

function norm() {
  var res, i;
  res = 0;
  for (i = 0; i < 10; i += 1) {
    res += Math.random()*2-1
  }
  return res;
}

var w = 500,
    h = 500;

var nodes = d3.range(400).map(function() {
  var true_y = (norm()*50)+250;
  return {
    radius: 4,
    y: true_y,
    true_x: 250,
    true_y: true_y }
  });

var force = d3.layout.force()
    .gravity(0)
    .charge(0)
    .friction(0.9)
    .nodes(nodes)
    .size([w, h]);

var root = nodes[0];
root.radius = 0;
root.fixed = true;

force.start();

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

svg.selectAll("circle")
    .data(nodes)
  .enter().append("svg:circle")
    .attr("r", function(d) { return d.radius; })
    .style("fill", 'steelblue')
    .style("stroke", "black");

force.on("tick", function(e) {
  var q,
    node,
    i = 0,
    n = nodes.length;

  var q = d3.geom.quadtree(nodes);

  while (++i < n) {
    node = nodes[i];
    q.visit(collide(node));
    xerr = node.x - node.true_x;
    yerr = node.y - node.true_y;
    node.x -= xerr*0.005;
    node.y -= yerr*0.8;
  }


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

function collide(node) {
  var r = node.radius,
    nx1,
    nx2,
    ny1,
    ny2,
    xerr,
    yerr;

  nx1 = node.x - r;
  nx2 = node.x + r;
  ny1 = node.y - r;
  ny2 = node.y + r;

  return function(quad, x1, y1, x2, y2) {
    if (quad.point && (quad.point !== node)) {
      var x = node.x - quad.point.x,
          y = node.y - quad.point.y,
          l = Math.sqrt(x * x + y * y),
          r = node.radius + quad.point.radius;
      if (l < r) {
        // we're colliding.
        var xnudge, ynudge, nudge_factor;
        nudge_factor = (l - r) / l * .4;
        xnudge = x*nudge_factor;
        ynudge = y*nudge_factor;
        node.x -= xnudge;
        node.y -= ynudge;
        quad.point.x += xnudge;
        quad.point.y += ynudge;
      }
    }
    return x1 > nx2
        || x2 < nx1
        || y1 > ny2
        || y2 < ny1;
  };
}

    </script>

  </body>
</html>