innerRadius (innerRadius) {
        if (!arguments.length) {
            return this._innerRadius;
        }
        this._innerRadius = innerRadius;
        return this;
    }
Get or set the inner radius of the pie chart. If the inner radius is greater than 0px then the
pie chart will be rendered as a doughnut chart.
@param {Number} [innerRadius=0]
@returns {Number|PieChart}