# [Matplotlib-users] logarithmic axis on colorbar

Jody Klymak jklymak at uvic.ca
Fri Sep 4 23:09:53 CEST 2015

```"""
Demonstration of using norm to map colormaps onto data in non-linear ways.
"""

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.colors as colors
from matplotlib.mlab import bivariate_normal

'''
Lognorm: Instead of pcolor log10(Z1) you can have colorbars that have
the exponential labels using a norm.
'''
N = 100
X, Y = np.mgrid[-3:3:complex(0, N), -2:2:complex(0, N)]

# A low hump with a spike coming out of the top right.  Needs to have
# z/colour axis on a log scale so we see both hump and spike.  linear
# scale only shows the spike.
Z1 = bivariate_normal(X, Y, 0.1, 0.2, 1.0, 1.0) +  \
0.1 * bivariate_normal(X, Y, 1.0, 1.0, 0.0, 0.0)

fig, ax = plt.subplots(2, 1)

pcm = ax[0].pcolor(X, Y, Z1,
norm=colors.LogNorm(vmin=Z1.min(), vmax=Z1.max()),
cmap='PuBu_r')
fig.colorbar(pcm, ax=ax[0], extend='max')

pcm = ax[1].pcolor(X, Y, Z1, cmap='PuBu_r')
fig.colorbar(pcm, ax=ax[1], extend='max')
fig.show()
This is not part of the release docs, but is on master, so should be available soon.  But basically you want to use `norm=colors.LogNorm()`

Cheers,   Jody

> On 4 Sep 2015, at  13:38 PM, Slavin, Jonathan <jslavin at cfa.harvard.edu> wrote:
>
> Hi all,
>
> I'm wanting to plot an image that is logarithmically scaled, and I'd like to have the associated colorbar ticks be logarithmic.  So, for example, say img is the image
> fig,ax = plt.subplots()
> cax = ax.imshow(np.log10(img))
> fig.colorbar(cax)
>
> gives me a colorbar with the tick labels with values that are the log10(img) values, whereas I want the colorbar to be labeled with a logarithmic axis and the img values.  How could I do that?  Any help would be appreciated.
>
> Thanks,
> Jon
>
> --
> ________________________________________________________
> Jonathan D. Slavin                 Harvard-Smithsonian CfA
> jslavin at cfa.harvard.edu <mailto:jslavin at cfa.harvard.edu>       60 Garden Street, MS 83
> phone: (617) 496-7981       Cambridge, MA 02138-1516
> cell: (781) 363-0035             USA
> ________________________________________________________
>
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--
Jody Klymak
http://web.uvic.ca/~jklymak/

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```