@classmethod
def arg_color_axis(cls, parser):
    """Add an `~argparse.ArgumentGroup` for colour-axis options.
        """
    group = parser.add_argument_group('Colour axis options')
    group.add_argument('--imin', type=float, help='minimum value for colorbar')
    group.add_argument('--imax', type=float, help='maximum value for colorbar')
    group.add_argument('--cmap', help=
        'Colormap. See https://matplotlib.org/examples/color/colormaps_reference.html for options'
        )
    group.add_argument('--color-scale', choices=('log', 'linear'), help=
        'scale for colorbar')
    group.add_argument('--norm', nargs='?', const='median', choices=(
        'median', 'mean'), metavar='NORM', help=
        'normalise each pixel against average in that frequency bin')
    group.add_argument('--nocolorbar', action='store_true', help=
        'hide the colour bar')
