code: def numRef_xml(self, wksht_ref, number_format, values):
          
          pt_xml = self.pt_xml(values)
          return (
              '            <c:numRef>\n'
              '              <c:f>{wksht_ref}</c:f>\n'
              '              <c:numCache>\n'
              '                <c:formatCode>{number_format}</c:formatCode>\n'
              '{pt_xml}'
              '              </c:numCache>\n'
              '            </c:numRef>\n'
          ).format(**{
              'wksht_ref':     wksht_ref,
              'number_format': number_format,
              'pt_xml':        pt_xml,
          })
docs: Return the ``<c:numRef>`` element specified by the parameters as
          unicode text.