public void addDomainMarker(Marker marker) {
        // defer argument checking...
        addDomainMarker(marker, Layer.FOREGROUND);
    }

    /**
     * Adds a marker for the domain axis in the specified layer and sends a
     * {@link PlotChangeEvent} to all registered listeners.
     * <P>
     * Typically a marker will be drawn by the renderer as a line perpendicular
     * to the range 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).
     *
     * @see #addDomainMarker(int, Marker, Layer)
     */
    public void addDomainMarker(Marker marker, Layer layer) {
        addDomainMarker(0, marker, layer);