dataOpacity (opacity) {
        if (!arguments.length) {
            return this._dataOpacity;
        }
        this._dataOpacity = opacity;
        return this;
    }
Get or set the opacity when rendering data.
@example
// If individual data points are rendered increase the opacity.
chart.dataOpacity(0.7);
@param {Number} [opacity=0.3]
@returns {Number|BoxPlot}