colors (colorScale) {
        if (!arguments.length) {
            return this._colors;
        }
        if (colorScale instanceof Array) {
            this._colors = scaleQuantize().range(colorScale); // deprecated legacy support, note: this fails for ordinal domains
        } else {
            this._colors = typeof colorScale === 'function' ? colorScale : utils.constant(colorScale);
        }
        return this;
    }
Retrieve current color scale or set a new color scale. This methods accepts any function that
operates like a d3 scale.
@memberof ColorMixin
@instance
@see {@link https://github.com/d3/d3-scale/blob/master/README.md d3.scale}
@example
// alternate categorical scale
chart.colors(d3.scale.category20b());
// ordinal scale
chart.colors(d3.scaleOrdinal().range(['red','green','blue']));
// convenience method, the same as above
chart.ordinalColors(['red','green','blue']);
// set a linear scale
chart.linearColors(["#4575b4", "#ffffbf", "#a50026"]);
@param {d3.scale} [colorScale=d3.scaleOrdinal(d3.schemeCategory20c)]
@returns {d3.scale|ColorMixin}