code: def xVal(self):
          
          xml = self._xVal_tmpl.format(**{
              'nsdecls':    ' %s' % nsdecls('c'),
              'numRef_xml': self.numRef_xml(
                  self._series.x_values_ref, self._series.number_format,
                  self._series.x_values
              ),
          })
          return parse_xml(xml)
docs: Return the ``<c:xVal>`` element for this series as an oxml element.
          This element contains the X values for this series.