canvas (canvasElement) {
        if (!arguments.length) {
            return this._canvas;
        }
        this._canvas = canvasElement;
        return this;
    }
Set or get canvas element. You should usually only ever use the get method as
dc.js will handle canvas element generation.  Provides valid canvas only when
{@link ScatterPlot#useCanvas useCanvas} is set to `true`
@param {CanvasElement|d3.selection} [canvasElement]
@return {CanvasElement|d3.selection}