constructor (parent, chartGroup) {
        super();

        this.transitionDuration(750);

        this.transitionDelay(0);

        this.anchor(parent, chartGroup);
    }
Create a Bubble Chart.

@example
// create a bubble chart under #chart-container1 element using the default global chart group
var bubbleChart1 = new BubbleChart('#chart-container1');
// create a bubble chart under #chart-container2 element using chart group A
var bubbleChart2 = new BubbleChart('#chart-container2', '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.