r (bubbleRadiusScale) {
        if (!arguments.length) {
            return this._r;
        }
        this._r = bubbleRadiusScale;
        return this;
    }
Get or set the bubble radius scale. By default the bubble chart uses
{@link https://github.com/d3/d3-scale/blob/master/README.md#scaleLinear d3.scaleLinear().domain([0, 100])}
as its radius scale.
@memberof BubbleMixin
@instance
@see {@link https://github.com/d3/d3-scale/blob/master/README.md d3.scale}
@param {d3.scale} [bubbleRadiusScale=d3.scaleLinear().domain([0, 100])]
@returns {d3.scale|BubbleMixin}