code: def val_xml(self):
          
          return self._val_tmpl.format(**{
              'nsdecls':       '',
              'values_ref':    self._series.values_ref,
              'number_format': self._series.number_format,
              'val_count':     len(self._series),
              'val_pt_xml':    self._val_pt_xml,
          })
docs: Return the unicode XML snippet for the ``<c:val>`` element describing
          this series, containing the series values and their spreadsheet range
          reference.