area.js

function d3_svg_area(projection) {
  var x0 = d3_svg_lineX,
      x1 = d3_svg_lineX,
      y0 = 0,
      y1 = d3_svg_lineY,
      defined = d3_true,
      interpolate = d3_svg_lineLinear,
      interpolateKey = interpolate.key,
      interpolateReverse = interpolate,
      L = "L",
      tension = .7;

  function area(data) {
    var segments = [],
        points0 = [],
        points1 = [],
        i = -1,
        n = data.length,
        d,
        fx0 = d3_functor(x0),
        fy0 = d3_functor(y0),
        fx1 = x0 === x1 ? function() { return x; } : d3_functor(x1),
        fy1 = y0 === y1 ? function() { return y; } : d3_functor(y1),
        x,
        y;

    function segment() {
      segments.push("M", interpolate(projection(points1), tension),
          L, interpolateReverse(projection(points0.reverse()), tension),
          "Z");
    }

    while (++i < n) {
      if (defined.call(this, d = data[i], i)) {
        points0.push([x = +fx0.call(this, d, i), y = +fy0.call(this, d, i)]);
        points1.push([+fx1.call(this, d, i), +fy1.call(this, d, i)]);
      } else if (points0.length) {
        segment();
        points0 = [];
        points1 = [];
      }
    }

    if (points0.length) segment();

    return segments.length ? segments.join("") : null;
  }

  area.x = function(_) {
    if (!arguments.length) return x1;
    x0 = x1 = _;
    return area;
  };

  area.x0 = function(_) {
    if (!arguments.length) return x0;
    x0 = _;
    return area;
  };

  area.x1 = function(_) {
    if (!arguments.length) return x1;
    x1 = _;
    return area;
  };

  area.y = function(_) {
    if (!arguments.length) return y1;
    y0 = y1 = _;
    return area;
  };

  area.y0 = function(_) {
    if (!arguments.length) return y0;
    y0 = _;
    return area;
  };

  area.y1 = function(_) {
    if (!arguments.length) return y1;
    y1 = _;
    return area;
  };

  area.defined  = function(_) {
    if (!arguments.length) return defined;
    defined = _;
    return area;
  };

  area.interpolate = function(_) {
    if (!arguments.length) return interpolateKey;
    if (typeof _ === "function") interpolateKey = interpolate = _;
    else interpolateKey = (interpolate = d3_svg_lineInterpolators.get(_) || d3_svg_lineLinear).key;
    interpolateReverse = interpolate.reverse || interpolate;
    L = interpolate.closed ? "M" : "L";
    return area;
  };

  area.tension = function(_) {
    if (!arguments.length) return tension;
    tension = _;
    return area;
  };

  return area;
}

d3_svg_lineStepBefore.reverse = d3_svg_lineStepAfter;
d3_svg_lineStepAfter.reverse = d3_svg_lineStepBefore;

d3.svg.area = function() {
  return d3_svg_area(d3_identity);
};