# [Matplotlib-users] logarithmic axis on colorbar

Slavin, Jonathan jslavin at cfa.harvard.edu
Wed Sep 9 16:35:03 CEST 2015

```Hi Jody,

Thanks for the pointer.  It looks like that should work nicely.  I am
curious though if there isn't a more general way to provide the data to be
used for a colorbar without creating a plot.  I guess you always need to
provide an Axes object.

Jon

On Fri, Sep 4, 2015 at 5:09 PM, Jody Klymak <jklymak at uvic.ca> wrote:

> """Demonstration of using norm to map colormaps onto data in non-linear ways."""
> import numpy as npimport matplotlib.pyplot as pltimport matplotlib.colors as colorsfrom matplotlib.mlab import bivariate_normal
> '''Lognorm: Instead of pcolor log10(Z1) you can have colorbars that havethe exponential labels using a norm.'''N = 100X, 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       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/
>
>
>
>
>
>

--
________________________________________________________
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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