maxBubbleRelativeSize (relativeSize) {
        if (!arguments.length) {
            return this._maxBubbleRelativeSize;
        }
        this._maxBubbleRelativeSize = relativeSize;
        return this;
    }
Get or set the maximum relative size of a bubble to the length of x axis. This value is useful
when the difference in radius between bubbles is too great.
@memberof BubbleMixin
@instance
@param {Number} [relativeSize=0.3]
@returns {Number|BubbleMixin}