/**
  * Updates the value of an item in the series and sends a {@link SeriesChangeEvent} to all
  * registered listeners.
  *
  * @param index the item (zero based index).
  * @param y the new value (<code>null</code> permitted).
  * @since 1.0.1
  */
 public void updateByIndex(int index, Number y) {
   update(index, y);
 }