code: def append(self, sc):
          
          ae = sc.aesthetics[0]
          cover_ae = self.find(ae)
          if any(cover_ae):
              warn(_TPL_DUPLICATE_SCALE.format(ae), PlotnineWarning)
              idx = cover_ae.index(True)
              self.pop(idx)
          # super() does not work well with reloads
          list.append(self, sc)
docs: Add scale 'sc' and remove any previous
          scales that cover the same aesthetics