code: def tx_xml(self):
          
          return self._tx_tmpl.format(**{
              'wksht_ref':   self._series.name_ref,
              'series_name': self.name,
              'nsdecls':     '',
          })
docs: Return the ``<c:tx>`` (tx is short for 'text') element for this
          series as unicode text. This element contains the series name.