code: public Visualization getVisualization(PlotItem item, VisualizationTask task) {
      Pair<Element, Visualization> pair = map.get(key(item, task));
      return pair == null ? null : pair.second;
    }
docs: Get the visualization referenced by a item/key combination.
  
  @param item Plot ttem
  @param task Visualization task
  @return Visualization