code: def _altair_hline_(self, xfield, yfield, opts, style, encode):
          
          try:
              rawy = yfield
              if ":" in yfield:
                  rawy = yfield.split(":")[0]
              mean = self.df[rawy].mean()
              l = []
              i = 0
              while i < len(self.df[rawy]):
                  l.append(mean)
                  i += 1
              self.df["Mean"] = l
              chart = Chart(self.df).mark_line(**style).encode(x=xfield, \
                                              y="Mean", **encode).properties(**opts)
              self.drop("Mean")
              return chart
          except Exception as e:
              self.err(e, "Can not draw mean line chart")
docs: Get a mean line chart