public void removeDataPointListener(int dataPointId, DataPointListener l) {
   DataPointListener listeners =
       DataPointEventMulticaster.remove(dataPointListeners.get(dataPointId), l);
   if (listeners == null) dataPointListeners.remove(dataPointId);
   else dataPointListeners.put(dataPointId, listeners);
 }
 public void addDataPointListener(int dataPointId, DataPointListener l) {
   DataPointListener listeners = dataPointListeners.get(dataPointId);
   dataPointListeners.put(dataPointId, DataPointEventMulticaster.add(listeners, l));
 }