code: def _cat_num_pt_xml(self):
          
          xml = ''
          for idx, category in enumerate(self._series.categories):
              xml += (
                  '                <c:pt idx="{cat_idx}">\n'
                  '                  <c:v>{cat_lbl_str}</c:v>\n'
                  '                </c:pt>\n'
              ).format(**{
                  'cat_idx':     idx,
                  'cat_lbl_str': category.numeric_str_val(self._date_1904),
              })
          return xml
docs: The unicode XML snippet for the ``<c:pt>`` elements when category
          labels are numeric (including date type).