Re: [Matplotlib-users] reverse axis on colorbar, and showing under- and over-colors

[Maarten S. <maa...@kn...>]

Hi,

I did solve my own question. For posterity, and perhaps for a more
elegant solution, I post my solution here.

On Tue, 2010-10-26 at 10:56 +0200, Maarten Sneep wrote:

> I have an image with cloud pressures, 1000 at the surface, 200 at the
> top of the atmosphere. I'd like to reverse the axis, i.e. 1000 at the
> bottom of the scale, 200 at the top. How should I approach this?

Instead of using a figure.colorbar() call, I instantiate ColorbarBase
directly. I needed an axes object anyway to use a single colorbar fro a
two-subplot figure. The ColorbarBase gets the reversed colormap compared
to the image.

fig = plt.figure()
norm=Normalize(vmin=200.0, vmax=1000.0, clip=False)
cmap = cm.jet
cmap.set_bad(color=(0.75,0.75, 0.75))
cmap.set_over(color=(1.0, 1.0, 1.0))
cmap.set_under(color=(0.5, 0.5, 0.5))
cmap_bar = cm.jet_r
# note that under and over are reversed.
cmap_bar.set_bad(color=(0.75,0.75, 0.75))
cmap_bar.set_under(color=(1.0, 1.0, 1.0))
cmap_bar.set_over(color=(0.5, 0.5, 0.5))

# make subplots
ax1 = fig.add_subplot(211)
ax1.imshow(cp1, origin='lower', cmap=cmap,
    interpolation='nearest', aspect='auto', norm=norm)
ax2 = fig.add_subplot(212)
ax2.imshow(cp2, origin='lower', cmap=cmap,
    interpolation='nearest', aspect='auto', norm=norm)

# make room for colorbar
plt.subplots_adjust(left=0.07, hspace=0.25, right=0.825)
cbar_ax = plt.axes([0.875, 0.1, 0.025, 0.8])
tickvals = [200, 400, 600, 800, 1000]
cbar = ColorbarBase(cbar_ax, cmap=cmap_bar, 
    ticks=tickvals, extend='both', norm=norm)
# I use latex formatting, original is more interesting.
labels = ['%d' % v for v in tickvals] 
cbar.ax.set_yticklabels(lblslabels[::-1])

Dirty: yes, and it only works because of the equidistant labels. A more
elegant solution is appreciated. 

> Related: I have set colors for under- and over-values. I'd like to
> display a small patch of these colors at either end of the scale. The
> bad values are clear enough.

extend='both' keyword to the ColorbarBase() call, or the colorbar()
call.

Maarten