The unicode XML snippet for the ``<c:cat>`` element for this series,
        containing the category labels and spreadsheet reference.

def cat_xml(self):
        """
        The unicode XML snippet for the ``<c:cat>`` element for this series,
        containing the category labels and spreadsheet reference.
        """
        categories = self._series.categories

        if categories.are_numeric:
            return self._numRef_cat_tmpl.format(**{
                'wksht_ref':     self._series.categories_ref,
                'number_format': categories.number_format,
                'cat_count':     categories.leaf_count,
                'cat_pt_xml':    self._cat_num_pt_xml,
                'nsdecls':       '',
            })

        if categories.depth == 1:
            return self._cat_tmpl.format(**{
                'wksht_ref':  self._series.categories_ref,
                'cat_count':  categories.leaf_count,
                'cat_pt_xml': self._cat_pt_xml,
                'nsdecls':    '',
            })

        return self._multiLvl_cat_tmpl.format(**{
            'wksht_ref': self._series.categories_ref,
            'cat_count': categories.leaf_count,
            'lvl_xml':   self._lvl_xml(categories),
            'nsdecls':   '',
        })