radiusValueAccessor (radiusValueAccessor) {
        if (!arguments.length) {
            return this._rValueAccessor;
        }
        this._rValueAccessor = radiusValueAccessor;
        return this;
    }
Get or set the radius value accessor function. If set, the radius value accessor function will
be used to retrieve a data value for each bubble. The data retrieved then will be mapped using
the r scale to the actual bubble radius. This allows you to encode a data dimension using bubble
size.
@memberof BubbleMixin
@instance
@param {Function} [radiusValueAccessor]
@returns {Function|BubbleMixin}