# colors

Hi all,

I’m trying to use imshow to plot some values which fall on the interval [0,1]. I need to

use a logscale to emphasize the scales of the data. The solution I found checking some discussions was like this

plt.imshow(X, interpolation=‘none’, norm=matplotlib.colors.LogNorm())

However, I notice that the way these colors are assigned are not always the same (although my data always contains the minimum value 0.0 and the maximum 1.0). I need to have a coherent color scale to indicate the real values. Is it easier to do the color code myself? What is the proper way of tackling this problem??

It’s pretty much the same problem described here, but with a logscale…

http://stackoverflow.com/questions/7875688/how-can-i-create-a-standard-colorbar-for-a-series-of-plots-in-python

Thank you very much!

Bruno

Hi all,

I'm trying to use imshow to plot some values which fall on the interval
[0,1]. I need to
use a logscale to emphasize the scales of the data. The solution I found
checking some discussions was like this

plt.imshow(X, interpolation='none', norm=matplotlib.colors.LogNorm())

However, I notice that the way these colors are assigned are not always
the same (although my data always contains the minimum value 0.0 and
the maximum 1.0). I need to have a coherent color scale to indicate
the real values. Is it easier to do the color code myself? What is the
proper way of tackling this problem??

Use the vmin and vmax kwargs to LogNorm, remembering that vmin must be greater than zero for a log scale.

Eric

···

On 2014/06/17, 8:59 AM, Bruno Pace wrote:

It's pretty much the same problem described here, but with a logscale...

http://stackoverflow.com/questions/7875688/how-can-i-create-a-standard-colorbar-for-a-series-of-plots-in-python

Thank you very much!

Bruno

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Ok, so using the norm=SymLogNorm I cannot distinguish the values that are exactly 0.0 from the really small ones, right? Would it be possible to make use of the set_bad method without having to use masked arrays, just combining the SymLogNorm and the set_bad?

Thanks!

···

2014-06-17 21:20 GMT+02:00 Eric Firing <efiring@…2611…2…>:

On 2014/06/17, 8:59 AM, Bruno Pace wrote:

Hi all,

I’m trying to use imshow to plot some values which fall on the interval

[0,1]. I need to

use a logscale to emphasize the scales of the data. The solution I found

checking some discussions was like this

plt.imshow(X, interpolation=‘none’, norm=matplotlib.colors.LogNorm())

However, I notice that the way these colors are assigned are not always

the same (although my data always contains the minimum value 0.0 and

the maximum 1.0). I need to have a coherent color scale to indicate

the real values. Is it easier to do the color code myself? What is the

proper way of tackling this problem??

Use the vmin and vmax kwargs to LogNorm, remembering that vmin must be

greater than zero for a log scale.

Eric

It’s pretty much the same problem described here, but with a logscale…

http://stackoverflow.com/questions/7875688/how-can-i-create-a-standard-colorbar-for-a-series-of-plots-in-python

Thank you very much!

Bruno

HPCC Systems Open Source Big Data Platform from LexisNexis Risk Solutions

Find What Matters Most in Your Big Data with HPCC Systems

Open Source. Fast. Scalable. Simple. Ideal for Dirty Data.

Leverages Graph Analysis for Fast Processing & Easy Data Exploration

http://p.sf.net/sfu/hpccsystems

Matplotlib-users mailing list

Matplotlib-users@lists.sourceforge.net

https://lists.sourceforge.net/lists/listinfo/matplotlib-users

HPCC Systems Open Source Big Data Platform from LexisNexis Risk Solutions

Find What Matters Most in Your Big Data with HPCC Systems

Open Source. Fast. Scalable. Simple. Ideal for Dirty Data.

Leverages Graph Analysis for Fast Processing & Easy Data Exploration

http://p.sf.net/sfu/hpccsystems

Matplotlib-users mailing list

Matplotlib-users@lists.sourceforge.net

https://lists.sourceforge.net/lists/listinfo/matplotlib-users

Ok, so using the norm=SymLogNorm I cannot distinguish the values that
are exactly 0.0 from the really small ones, right? Would it be possible

Correct, the scale is linear for small values.

to make use of the set_bad method without having to use masked arrays,
just combining the SymLogNorm and the set_bad?

No, the mask is what identifies a point as bad. If you want to distinguish zero from non-zero, no matter how small, then this is the way to do it.

Now you have a masked array where the points that are exactly zero are masked.

The bad color won't show up on the colorbar, however. There is no suitable place for it. It can show only the range from vmin to vmax, and a "set_over" color for values greater than vmax, and a "set_under" color for values less than vmin.

Eric

···

On 2014/06/18, 5:23 AM, Bruno Pace wrote:

Ok! I’m getting there! I’ve been trying to figure out, though, how to set black - for example - for the zero values BUT interpolate also the colors linearly from black to blue in the linear region (from zero to the linear threshold). Is there a way to change the colormap like that?

Thanks a lot!

Ok, so using the norm=SymLogNorm I cannot distinguish the values that

are exactly 0.0 from the really small ones, right? Would it be possible

Correct, the scale is linear for small values.

to make use of the set_bad method without having to use masked arrays,

just combining the SymLogNorm and the set_bad?

No, the mask is what identifies a point as bad. If you want to distinguish zero from non-zero, no matter how small, then this is the way to do it.