d3.svg.diagonal = function() {

  var source = d3_svg_chordSource,

      target = d3_svg_chordTarget,

      projection = d3_svg_diagonalProjection;

  function diagonal(d, i) {

    var p0 = source.call(this, d, i),

        p3 = target.call(this, d, i),

        m = (p0.y + p3.y) / 2,

        p = [p0, {x: p0.x, y: m}, {x: p3.x, y: m}, p3];

    p = p.map(projection);

    return "M" + p[0] + "C" + p[1] + " " + p[2] + " " + p[3];

  }

  diagonal.source = function(x) {

    if (!arguments.length) return source;

    source = d3.functor(x);

    return diagonal;

  };

  diagonal.target = function(x) {

    if (!arguments.length) return target;

    target = d3.functor(x);

    return diagonal;

  };

  diagonal.projection = function(x) {

    if (!arguments.length) return projection;

    projection = x;

    return diagonal;

  };

  return diagonal;

};

function d3_svg_diagonalProjection(d) {

  return [d.x, d.y];

}