constructor (parent, chartGroup) {
        super();

        this._formatNumber = format(',d');
        this._crossfilter = null;
        this._groupAll = null;
        this._html = {some: '', all: ''};

        this._mandatoryAttributes(['crossfilter', 'groupAll']);

        this.anchor(parent, chartGroup);
    }
Create a Data Count widget.
@example
var ndx = crossfilter(data);
var all = ndx.groupAll();

new DataCount('.dc-data-count')
    .crossfilter(ndx)
    .groupAll(all);
@param {String|node|d3.selection} parent - Any valid
{@link https://github.com/d3/d3-selection/blob/master/README.md#select d3 single selector} specifying
a dom block element such as a div; or a dom element or d3 selection.
@param {String} [chartGroup] - The name of the chart group this chart instance should be placed in.
Interaction with a chart will only trigger events and redraws within the chart's group.