chart (chartFunction) {
        if (!arguments.length) {
            return this._chartFunction;
        }
        this._chartFunction = chartFunction;
        this._resetChildren();
        return this;
    }
Get or set the chart function, which generates the child charts.
@example
// put curve on the line charts used for the series
chart.chart(function(c) { return new LineChart(c).curve(d3.curveBasis); })
// do a scatter series chart
chart.chart(anchor => new ScatterPlot(anchor))
@param {Function} [chartFunction= (anchor) =>  new LineChart(anchor)]
@returns {Function|SeriesChart}