index.html

## index.html
<!DOCTYPE html>
<meta charset="utf-8">
<style>

.arc {
  fill-opacity: .2;
}

.axis .fill {
  stroke: #aaa;
  stroke-width: 8px;
  stroke-linecap: square;
}

.axis .stroke {
  stroke: #fff;
  stroke-width: 12px;
  stroke-linecap: square;
}

</style>
<body>
<script src="http://mbostock.github.com/d3/d3.v2.js?2.8.1"></script>
<script src="link.js"></script>
<script>

var width = 960,
    height = 500,
    innerRadius = 40,
    outerRadius = 240;

var angle = d3.scale.ordinal().domain(d3.range(4)).rangePoints([0, 2 * Math.PI]),
    radius = d3.scale.linear().range([innerRadius, outerRadius]),
    color = d3.scale.category20c().domain(d3.range(20));

var links = [
  {source: {x: 0, y0: 0.9, y1: 1.0}, target: {x: 1, y0: 0.5, y1: 1.0}, group:  0},
  {source: {x: 0, y0: 0.7, y1: 0.9}, target: {x: 1, y0: 0.4, y1: 0.5}, group:  1},
  {source: {x: 0, y0: 0.4, y1: 0.7}, target: {x: 1, y0: 0.2, y1: 0.4}, group:  2},
  {source: {x: 0, y0: 0.0, y1: 0.4}, target: {x: 1, y0: 0.0, y1: 0.2}, group:  3},
  {source: {x: 1, y0: 0.8, y1: 1.0}, target: {x: 2, y0: 0.5, y1: 1.0}, group:  4},
  {source: {x: 1, y0: 0.5, y1: 0.8}, target: {x: 2, y0: 0.2, y1: 0.5}, group:  5},
  {source: {x: 1, y0: 0.3, y1: 0.5}, target: {x: 2, y0: 0.1, y1: 0.2}, group:  6},
  {source: {x: 1, y0: 0.0, y1: 0.3}, target: {x: 2, y0: 0.0, y1: 0.1}, group:  7},
  {source: {x: 2, y0: 0.8, y1: 1.0}, target: {x: 0, y0: 0.5, y1: 1.0}, group:  8},
  {source: {x: 2, y0: 0.5, y1: 0.8}, target: {x: 0, y0: 0.2, y1: 0.5}, group:  9},
  {source: {x: 2, y0: 0.1, y1: 0.5}, target: {x: 0, y0: 0.1, y1: 0.2}, group: 10},
  {source: {x: 2, y0: 0.0, y1: 0.1}, target: {x: 0, y0: 0.0, y1: 0.1}, group: 11}
];

var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height)
  .append("g")
    .attr("transform", "translate(" + width / 2 + "," + height / 2 + ")");

svg.selectAll(".link")
    .data(links)
  .enter().append("path")
    .attr("class", "link")
    .attr("d", link()
    .angle(function(d) { return angle(d.x); })
    .startRadius(function(d) { return radius(d.y0); })
    .endRadius(function(d) { return radius(d.y1); }))
    .style("fill", function(d) { return color(d.group); });

svg.selectAll(".axis")
    .data(d3.range(3))
  .enter().append("g")
    .attr("class", "axis")
    .attr("transform", function(d) { return "rotate(" + degrees(angle(d)) + ")"; })
  .selectAll("line")
    .data(["stroke", "fill"])
  .enter().append("line")
    .attr("class", function(d) { return d; })
    .attr("x1", radius.range()[0])
    .attr("x2", radius.range()[1]);

function degrees(radians) {
  return radians / Math.PI * 180 - 90;
}

</script>

## link.js
function link() {
  var source = function(d) { return d.source; },
      target = function(d) { return d.target; },
      angle = function(d) { return d.angle; },
      startRadius = function(d) { return d.radius; },
      endRadius = startRadius,
      arcOffset = -Math.PI / 2;

  function link(d, i) {
    var s = node(source, this, d, i),
        t = node(target, this, d, i),
        x;
    if (t.a < s.a) x = t, t = s, s = x;
    if (t.a - s.a > Math.PI) s.a += 2 * Math.PI;
    var a1 = s.a + (t.a - s.a) / 3,
        a2 = t.a - (t.a - s.a) / 3;
    return "M" + Math.cos(s.a) * s.r0 + "," + Math.sin(s.a) * s.r0
        + "L" + Math.cos(s.a) * s.r1 + "," + Math.sin(s.a) * s.r1
        + "C" + Math.cos(a1) * s.r1 + "," + Math.sin(a1) * s.r1
        + " " + Math.cos(a2) * t.r1 + "," + Math.sin(a2) * t.r1
        + " " + Math.cos(t.a) * t.r1 + "," + Math.sin(t.a) * t.r1
        + "L" + Math.cos(t.a) * t.r0 + "," + Math.sin(t.a) * t.r0
        + "C" + Math.cos(a2) * t.r0 + "," + Math.sin(a2) * t.r0
        + " " + Math.cos(a1) * s.r0 + "," + Math.sin(a1) * s.r0
        + " " + Math.cos(s.a) * s.r0 + "," + Math.sin(s.a) * s.r0;
  }

  function node(method, thiz, d, i) {
    var node = method.call(thiz, d, i),
        a = +(typeof angle === "function" ? angle.call(thiz, node, i) : angle) + arcOffset,
        r0 = +(typeof startRadius === "function" ? startRadius.call(thiz, node, i) : startRadius),
        r1 = (startRadius === endRadius ? r0 : +(typeof endRadius === "function" ? endRadius.call(thiz, node, i) : endRadius));
    return {r0: r0, r1: r1, a: a};
  }

  // Expose selected locals
  ["source","target","angle","startRadius","endRadius"].forEach(function(fn) {link[fn] =  function(_){if(!arguments){return eval(fn)};eval(fn+"=_");return link}})

  return link;
}