constructor () {
        // chartGroup:string => charts:array
        this._chartMap = {};
    }
The ChartRegistry maintains sets of all instantiated dc.js charts under named groups
and the default group. There is a single global ChartRegistry object named `chartRegistry`

A chart group often corresponds to a crossfilter instance. It specifies
the set of charts which should be updated when a filter changes on one of the charts or when the
global functions {@link filterAll filterAll}, {@link refocusAll refocusAll},
{@link renderAll renderAll}, {@link redrawAll redrawAll}, or chart functions
{@link baseMixin#renderGroup baseMixin.renderGroup},
{@link baseMixin#redrawGroup baseMixin.redrawGroup} are called.