code: def make_bands(self, ax):
          
          y_vals, y_prev, is_zero = [0], None, False
          prev_color_index = 0
          for plotter in self.plotters.values():
              for y, *_, color in plotter.iterator():
                  if self.colors.index(color) < prev_color_index:
                      if not is_zero and y_prev is not None:
                          y_vals.append((y + y_prev) * 0.5)
                          is_zero = True
                  else:
                      is_zero = False
                  prev_color_index = self.colors.index(color)
                  y_prev = y
  
          offset = plotter.group_offset  # pylint: disable=undefined-loop-variable
  
          y_vals.append(y_prev + offset)
          for idx, (y_start, y_stop) in enumerate(pairwise(y_vals)):
              ax.axhspan(y_start, y_stop, color="k", alpha=0.1 * (idx % 2))
          return ax
docs: Draw shaded horizontal bands for each plotter.