*
* @return The edge (never <code>null</code>).
*/
public RectangleEdge getDomainAxisEdge(final int index) {
RectangleEdge result = null;
final AxisLocation location = this.getDomainAxisLocation(index);
if (location != null) {
result = Plot.resolveDomainAxisLocation(location, this.orientation);
} else {
result = RectangleEdge.opposite(this.getDomainAxisEdge(0));
}