def depth(self):
        """
        Return an integer representing the number of hierarchical levels in
        this category collection. Returns 1 for non-hierarchical categories
        and 0 if no categories are present (generally meaning no series are
        present).
        """
        cat = self._xChart.cat
        if cat is None:
            return 0
        if cat.multiLvlStrRef is None:
            return 1
        return len(cat.lvls)