gap (gap) {
        if (!arguments.length) {
            return this._gap;
        }
        this._gap = gap;
        return this;
    }
Manually set fixed gap (in px) between bars instead of relying on the default auto-generated
gap.  By default the bar chart implementation will calculate and set the gap automatically
based on the number of data points and the length of the x axis.
@param {Number} [gap=2]
@returns {Number|BarChart}