public double getGap() {
        return this.gap;
    }

    /**
     * Sets the amount of space between subplots and sends a
     * {@link PlotChangeEvent} to all registered listeners.
     *
     * @param gap  the gap between subplots (in Java2D units).
     */
    public void setGap(double gap) {
        this.gap = gap;
        fireChangeEvent();
    }

    /**
     * Adds a subplot to the combined chart and sends a {@link PlotChangeEvent}
     * to all registered listeners.
     * <br><br>
     * The domain axis for the subplot will be set to <code>null</code>.  You
     * must ensure that the subplot has a non-null range axis.
     *
     * @param subplot  the subplot (<code>null</code> not permitted).
     */
    public void add(CategoryPlot subplot) {
        add(subplot, 1);