public void addDomainMarker(CategoryMarker marker) {
        addDomainMarker(marker, Layer.FOREGROUND);
    }

    /**
     * Adds a marker for display against the domain axis and sends a
     * {@link PlotChangeEvent} to all registered listeners.  Typically a marker
     * will be drawn by the renderer as a line perpendicular to the domain
     * axis, however this is entirely up to the renderer.
     *
     * @param marker  the marker (<code>null</code> not permitted).
     * @param layer  the layer (foreground or background) (<code>null</code>
     *               not permitted).
     *
     * @see #removeDomainMarker(Marker, Layer)
     */
    public void addDomainMarker(CategoryMarker marker, Layer layer) {
        addDomainMarker(0, marker, layer);