register (chart, group) {
        const _chartMap = this._chartMap;
        group = this._initializeChartGroup(group);
        _chartMap[group].push(chart);
    }
Add given chart instance to the given group, creating the group if necessary.
If no group is provided, the default group `constants.DEFAULT_CHART_GROUP` will be used.
@param {Object} chart dc.js chart instance
@param {String} [group] Group name
@return {undefined}