code: def levels(self):
          
          cat = self._xChart.cat
          if cat is None:
              return []
          return [CategoryLevel(lvl) for lvl in cat.lvls]
docs: Return a sequence of |CategoryLevel| objects representing the
          hierarchy of this category collection. The sequence is empty when the
          category collection is not hierarchical, that is, contains only
          leaf-level categories. The levels are ordered from the leaf level to
          the root level; so the first level will contain the same categories
          as this category collection.