Draw shaded horizontal bands for each plotter.

def make_bands(self, ax):
        """Draw shaded horizontal bands for each plotter."""
        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