d3.svg.symbol = function() {

  var type = d3_svg_symbolType,

      size = d3_svg_symbolSize;

  function symbol(d, i) {

    return (d3_svg_symbols[type.call(this, d, i)]

        || d3_svg_symbols.circle)

        (size.call(this, d, i));

  }

  symbol.type = function(x) {

    if (!arguments.length) return type;

    type = d3.functor(x);

    return symbol;

  };

  // size of symbol in square pixels

  symbol.size = function(x) {

    if (!arguments.length) return size;

    size = d3.functor(x);

    return symbol;

  };

  return symbol;

};

function d3_svg_symbolSize() {

  return 64;

}

function d3_svg_symbolType() {

  return "circle";

}

// TODO cross-diagonal?

var d3_svg_symbols = {

  "circle": function(size) {

    var r = Math.sqrt(size / Math.PI);

    return "M0," + r

        + "A" + r + "," + r + " 0 1,1 0," + (-r)

        + "A" + r + "," + r + " 0 1,1 0," + r

        + "Z";

  },

  "cross": function(size) {

    var r = Math.sqrt(size / 5) / 2;

    return "M" + -3 * r + "," + -r

        + "H" + -r

        + "V" + -3 * r

        + "H" + r

        + "V" + -r

        + "H" + 3 * r

        + "V" + r

        + "H" + r

        + "V" + 3 * r

        + "H" + -r

        + "V" + r

        + "H" + -3 * r

        + "Z";

  },

  "diamond": function(size) {

    var ry = Math.sqrt(size / (2 * d3_svg_symbolTan30)),

        rx = ry * d3_svg_symbolTan30;

    return "M0," + -ry

        + "L" + rx + ",0"

        + " 0," + ry

        + " " + -rx + ",0"

        + "Z";

  },

  "square": function(size) {

    var r = Math.sqrt(size) / 2;

    return "M" + -r + "," + -r

        + "L" + r + "," + -r

        + " " + r + "," + r

        + " " + -r + "," + r

        + "Z";

  },

  "triangle-down": function(size) {

    var rx = Math.sqrt(size / d3_svg_symbolSqrt3),

        ry = rx * d3_svg_symbolSqrt3 / 2;

    return "M0," + ry

        + "L" + rx +"," + -ry

        + " " + -rx + "," + -ry

        + "Z";

  },

  "triangle-up": function(size) {

    var rx = Math.sqrt(size / d3_svg_symbolSqrt3),

        ry = rx * d3_svg_symbolSqrt3 / 2;

    return "M0," + -ry

        + "L" + rx +"," + ry

        + " " + -rx + "," + ry

        + "Z";

  }

};

d3.svg.symbolTypes = d3.keys(d3_svg_symbols);

var d3_svg_symbolSqrt3 = Math.sqrt(3),

    d3_svg_symbolTan30 = Math.tan(30 * Math.PI / 180);