code: def cat_xml(self):
          
          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':   '',
          })
docs: The unicode XML snippet for the ``<c:cat>`` element for this series,
          containing the category labels and spreadsheet reference.