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.

def flattened_labels(self):
        """
        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.
        """
        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()]
        )