public RectangleInsets getAxisOffset() {
        return this.axisOffset;
    }

    /**
     * Sets the axis offsets (gap between the data area and the axes) and
     * sends a {@link PlotChangeEvent} to all registered listeners.
     *
     * @param offset  the offset (<code>null</code> not permitted).
     *
     * @see #getAxisOffset()
     */
    public void setAxisOffset(RectangleInsets offset) {
        if (offset == null) {
            throw new IllegalArgumentException("Null 'offset' argument.");
        }
        this.axisOffset = offset;
        fireChangeEvent();
    }

    /**
     * Returns the domain axis for the plot.  If the domain axis for this plot
     * is <code>null</code>, then the method will return the parent plot's
     * domain axis (if there is a parent plot).
     *
     * @return The domain axis (<code>null</code> permitted).
     *
     * @see #setDomainAxis(CategoryAxis)
     */
    public CategoryAxis getDomainAxis() {
        return getDomainAxis(0);
    }

    /**
     * Returns a domain axis.
     *
     * @param index  the axis index.
     *
     * @return The axis (<code>null</code> possible).
     *
     * @see #setDomainAxis(int, CategoryAxis)
     */
    public CategoryAxis getDomainAxis(int index) {
        CategoryAxis result = null;
        if (index < this.domainAxes.size()) {
            result = (CategoryAxis) this.domainAxes.get(index);
        }
        if (result == null) {
            Plot parent = getParent();
            if (parent instanceof CategoryPlot) {
                CategoryPlot cp = (CategoryPlot) parent;
                result = cp.getDomainAxis(index);
            }
        }
        return result;
    }

    /**
     * Sets the domain axis for the plot and sends a {@link PlotChangeEvent} to
     * all registered listeners.
     *
     * @param axis  the axis (<code>null</code> permitted).
     *
     * @see #getDomainAxis()
     */
    public void setDomainAxis(CategoryAxis axis) {
        setDomainAxis(0, axis);
    }

    /**
     * Sets a domain axis and sends a {@link PlotChangeEvent} to all
     * registered listeners.
     *
     * @param index  the axis index.
     * @param axis  the axis (<code>null</code> permitted).
     *
     * @see #getDomainAxis(int)
     */
    public void setDomainAxis(int index, CategoryAxis axis) {
        setDomainAxis(index, axis, true);