code: def _altair_line_num_(self, xfield, yfield, opts, style, encode):
          
          try:
              c = self._altair_chart_num_("line", xfield,
                                             yfield, opts, style, encode)
          except Exception as e:
              self.err(e, "Can not draw a line num chart")
              return
          return c
docs: Get a line + text number chart