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.

def levels(self):
        """
        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.
        """
        cat = self._xChart.cat
        if cat is None:
            return []
        return [CategoryLevel(lvl) for lvl in cat.lvls]