cubehelix.js

(function() {
  var radians = Math.PI / 180;

  d3.scale.cubehelix = function() {
    return d3.scale.linear()
        .range([d3.hsl(300, .6, 0), d3.hsl(-240, .6, 1)])
        .interpolate(d3.interpolateCubehelix);
  };

  d3.interpolateCubehelix = d3_interpolateCubehelix(1);
  d3.interpolateCubehelix.gamma = d3_interpolateCubehelix;

  function d3_interpolateCubehelix(γ) {
    return function(a, b) {
      a = d3.hsl(a);
      b = d3.hsl(b);

      var ah = (a.h + 120) * radians,
          bh = (b.h + 120) * radians - ah,
          as = a.s,
          bs = b.s - as,
          al = a.l,
          bl = b.l - al;

      if (isNaN(bs)) bs = 0, as = isNaN(as) ? b.s : as;
      if (isNaN(bh)) bh = 0, ah = isNaN(ah) ? b.h : ah;

      return function(t) {
        var h = ah + bh * t,
            l = Math.pow(al + bl * t, γ),
            a = (as + bs * t) * l * (1 - l);
        return "#"
            + hex(l + a * (-0.14861 * Math.cos(h) + 1.78277 * Math.sin(h)))
            + hex(l + a * (-0.29227 * Math.cos(h) - 0.90649 * Math.sin(h)))
            + hex(l + a * (+1.97294 * Math.cos(h)));
      };
    };
  }

  function hex(v) {
    var s = (v = v <= 0 ? 0 : v >= 1 ? 255 : v * 255 | 0).toString(16);
    return v < 0x10 ? "0" + s : s;
  }
})();