boxPadding (padding) {
        if (!arguments.length) {
            return this._rangeBandPadding();
        }
        return this._rangeBandPadding(padding);
    }
Get or set the spacing between boxes as a fraction of box size. Valid values are within 0-1.
See the {@link https://github.com/d3/d3-scale/blob/master/README.md#scaleBand d3 docs}
for a visual description of how the padding is applied.
@see {@link https://github.com/d3/d3-scale/blob/master/README.md#scaleBand d3.scaleBand}
@param {Number} [padding=0.8]
@returns {Number|BoxPlot}