externalLabels (externalLabelRadius) {
        if (arguments.length === 0) {
            return this._externalLabelRadius;
        } else if (externalLabelRadius) {
            this._externalLabelRadius = externalLabelRadius;
        } else {
            this._externalLabelRadius = undefined;
        }

        return this;
    }
Position slice labels offset from the outer edge of the chart.

The argument specifies the extra radius to be added for slice labels.
@param {Number} [externalLabelRadius]
@returns {Number|PieChart}