Return the ``<c:tx>`` (tx is short for 'text') element for this
        series as unicode text. This element contains the series name.

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