deregister (chart, group) {
        group = this._initializeChartGroup(group);
        for (let i = 0; i < (this._chartMap)[group].length; i++) {
            if ((this._chartMap)[group][i].anchorName() === chart.anchorName()) {
                (this._chartMap)[group].splice(i, 1);
                break;
            }
        }
    }
Remove given chart instance from 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}