chartGroup (chartGroup) {
        if (!arguments.length) {
            return this._chartGroup;
        }
        if (!this._isChild) {
            deregisterChart(this, this._chartGroup);
        }
        this._chartGroup = chartGroup;
        if (!this._isChild) {
            registerChart(this, this._chartGroup);
        }
        return this;
    }
Get or set the chart group to which this chart belongs. Chart groups are rendered or redrawn
together since it is expected they share the same underlying crossfilter data set.
@param {String} [chartGroup]
@returns {String|BaseMixin}