code: def flattened_labels(self):
          
          cat = self._xChart.cat
          if cat is None:
              return ()
  
          if cat.multiLvlStrRef is None:
              return tuple([(category.label,) for category in self])
  
          return tuple(
              [tuple([category.label for category in reversed(flat_cat)])
               for flat_cat in self._iter_flattened_categories()]
          )
docs: Return a sequence of tuples, each containing the flattened hierarchy
          of category labels for a leaf category. Each tuple is in parent ->
          child order, e.g. ``('US', 'CA', 'San Francisco')``, with the leaf
          category appearing last. If this categories collection is
          non-hierarchical, each tuple will contain only a leaf category label.
          If the plot has no series (and therefore no categories), an empty
          tuple is returned.