docs: def plot_chmap(cube, kidid, ax=None, **kwargs):
      """Plot an intensity map.
  
      Args:
          cube (xarray.DataArray): Cube which the spectrum information is included.
          kidid (int): Kidid.
          ax (matplotlib.axes): Axis the figure is plotted on.
          kwargs (optional): Plot options passed to ax.imshow().
      """
      if ax is None:
          ax = plt.gca()
  
      index = np.where(cube.kidid == kidid)[0]
      if len(index) == 0:
          raise KeyError('Such a kidid does not exist.')
      index = int(index)
  
      im = ax.pcolormesh(cube.x, cube.y, cube[:, :, index].T, **kwargs)
      ax.set_xlabel('x')
      ax.set_ylabel('y')
      ax.set_title('intensity map ch #{}'.format(kidid))
      return im