constructor (parent, chartGroup) {
        super();

        this.colorAccessor(d => d || 0);

        this._geoPath = geoPath();
        this._projectionFlag = undefined;
        this._projection = undefined;

        this._geoJsons = [];

        this.anchor(parent, chartGroup);
    }
Create a Geo Choropleth Chart.
@example
// create a choropleth chart under '#us-chart' element using the default global chart group
var chart1 = new GeoChoroplethChart('#us-chart');
// create a choropleth chart under '#us-chart2' element using chart group A
var chart2 = new CompositeChart('#us-chart2', 'chartGroupA');
@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.