# logarithmic axis on colorbar

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 60 Garden Street, MS 83
phone: (617) 496-7981 Cambridge, MA 02138-1516
cell: (781) 363-0035 USA
________________________________________________________
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"""
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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Matplotlib-users at python.org
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--
Jody Klymak
http://web.uvic.ca/~jklymak/

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