constructor (parent, chartGroup) {
        super();

        this._formatNumber = format('.2s');
        this._html = {one: '', some: '', none: ''};
        this._lastValue = undefined;
        this._ariaLiveRegion = false;

        // dimension not required
        this._mandatoryAttributes(['group']);

        // default to ordering by value, to emulate old group.top(1) behavior when multiple groups
        this.ordering(kv => kv.value);

        this.data(group => {
            const valObj = group.value ? group.value() : this._maxBin(group.all());
            return this.valueAccessor()(valObj);
        });

        this.transitionDuration(250); // good default
        this.transitionDelay(0);

        this.anchor(parent, chartGroup);
    }
Create a Number Display widget.

@example
// create a number display under #chart-container1 element using the default global chart group
var display1 = new NumberDisplay('#chart-container1');
@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.