Return a ``<c:tx>`` oxml element for this series, containing the
        series name.

def tx(self):
        """
        Return a ``<c:tx>`` oxml element for this series, containing the
        series name.
        """
        xml = self._tx_tmpl.format(**{
            'wksht_ref':   self._series.name_ref,
            'series_name': self.name,
            'nsdecls':     ' %s' % nsdecls('c'),
        })
        return parse_xml(xml)