code: def _cat_pt_xml(self):
          
          xml = ''
          for idx, category in enumerate(self._series.categories):
              xml += (
                  '                <c:pt idx="{cat_idx}">\n'
                  '                  <c:v>{cat_label}</c:v>\n'
                  '                </c:pt>\n'
              ).format(**{
                  'cat_idx':   idx,
                  'cat_label': escape(to_unicode(category.label)),
              })
          return xml
docs: The unicode XML snippet for the ``<c:pt>`` elements containing the
          category names for this series.