The ``<c:val>`` XML for this series, as an oxml element.

def val(self):
        """
        The ``<c:val>`` XML for this series, as an oxml element.
        """
        xml = self._val_tmpl.format(**{
            'nsdecls':       ' %s' % nsdecls('c'),
            'values_ref':    self._series.values_ref,
            'number_format': self._series.number_format,
            'val_count':     len(self._series),
            'val_pt_xml':    self._val_pt_xml,
        })
        return parse_xml(xml)