renderArea (renderArea) {
        if (!arguments.length) {
            return this._renderArea;
        }
        this._renderArea = renderArea;
        return this;
    }
Get or set render area flag. If the flag is set to true then the chart will render the area
beneath each line and the line chart effectively becomes an area chart.
@param  {Boolean} [renderArea=false]
@returns {Boolean|LineChart}