d3.svg.chord = function() {

  var source = d3_svg_chordSource,

      target = d3_svg_chordTarget,

      radius = d3_svg_chordRadius,

      startAngle = d3_svg_arcStartAngle,

      endAngle = d3_svg_arcEndAngle;

  // TODO Allow control point to be customized.

  function chord(d, i) {

    var s = subgroup(this, source, d, i),

        t = subgroup(this, target, d, i);

    return "M" + s.p0

      + arc(s.r, s.p1) + (equals(s, t)

      ? curve(s.r, s.p1, s.r, s.p0)

      : curve(s.r, s.p1, t.r, t.p0)

      + arc(t.r, t.p1)

      + curve(t.r, t.p1, s.r, s.p0))

      + "Z";

  }

  function subgroup(self, f, d, i) {

    var subgroup = f.call(self, d, i),

        r = radius.call(self, subgroup, i),

        a0 = startAngle.call(self, subgroup, i) + d3_svg_arcOffset,

        a1 = endAngle.call(self, subgroup, i) + d3_svg_arcOffset;

    return {

      r: r,

      a0: a0,

      a1: a1,

      p0: [r * Math.cos(a0), r * Math.sin(a0)],

      p1: [r * Math.cos(a1), r * Math.sin(a1)]

    };

  }

  function equals(a, b) {

    return a.a0 == b.a0 && a.a1 == b.a1;

  }

  function arc(r, p) {

    return "A" + r + "," + r + " 0 0,1 " + p;

  }

  function curve(r0, p0, r1, p1) {

    return "Q 0,0 " + p1;

  }

  chord.radius = function(v) {

    if (!arguments.length) return radius;

    radius = d3.functor(v);

    return chord;

  };

  chord.source = function(v) {

    if (!arguments.length) return source;

    source = d3.functor(v);

    return chord;

  };

  chord.target = function(v) {

    if (!arguments.length) return target;

    target = d3.functor(v);

    return chord;

  };

  chord.startAngle = function(v) {

    if (!arguments.length) return startAngle;

    startAngle = d3.functor(v);

    return chord;

  };

  chord.endAngle = function(v) {

    if (!arguments.length) return endAngle;

    endAngle = d3.functor(v);

    return chord;

  };

  return chord;

};

function d3_svg_chordSource(d) {

  return d.source;

}

function d3_svg_chordTarget(d) {

  return d.target;

}

function d3_svg_chordRadius(d) {

  return d.radius;

}

function d3_svg_chordStartAngle(d) {

  return d.startAngle;

}

function d3_svg_chordEndAngle(d) {

  return d.endAngle;

}