Custom continuous color map in matplotlib

Question

I have read some questions on this subject but I have been unable to find a specific answer to my question.

Let consider the image below:

Color map example

My goal is just to change the limit colors of the map, e.g. in this case the color map goes from dark red to dark blue, let's say I would like it to go from dark green to dark blue. Specifically, I would it to go from colors #244162 to #DCE6F1 (tonalities of blue) in the same continuous way as in the example above.

How is it possible to do this?

[EDIT]

I have tried the following code:

import matplotlib.pyplot as plt
import matplotlib.colors as clr

some_matrix = ...
cmap = clr.LinearSegmentedColormap('custom blue', ['#244162','#DCE6F1'], N=256)
plt.matshow(some_matrix, cmap=cmap)

But I get the error message TypeError: list indices must be integers, not unicode.

Other than "read some questions", have you tried anything? For example, have you looked for an existing colormap that would work or tried LinearSegmentedColormap? — tom10
– tom10, Commented May 23, 2016 at 23:05
Here are some specific examples in matplotlib documentation. matplotlib.org/examples/color/colormaps_reference.html — Kapil Sharma
– Kapil Sharma, Commented May 23, 2016 at 23:11
I have tried some existing color maps but I would like to define a custom one to keep a uniform formatting throughout a document. I have tried to implement LinearSegmentedColormap with this code cmap = matplotlib.colors.LinearSegmentedColormap('custom blue', ['#244162','#DCE6F1'], N=256) but it hasn't worked. — Daneel Olivaw
– Daneel Olivaw, Commented May 23, 2016 at 23:20
@DaneelOlivaw put that information in the question itself, and describe exactly what "it hasn't worked" means. Did the cm not generate? Was it not the colors you were looking for? What happened? — MattDMo
– MattDMo, Commented May 23, 2016 at 23:36

tmdavison · Accepted Answer · 2016-05-24 12:27:56Z

18

LinearSegmentedColormap doesn't take a list of colours, it takes the following argument:

a dictionary with a red, green and blue entries. Each entry should be a list of x, y0, y1 tuples, forming rows in a table. Entries for alpha are optional.

So, you either need to define a dictionary as above, or in your case, I think you just want to use the LinearSegmentedColormap.from_list() method:

import matplotlib.pyplot as plt
import matplotlib.colors as clr
import numpy as np

some_matrix = np.random.rand(10,10)

cmap = clr.LinearSegmentedColormap.from_list('custom blue', ['#244162','#DCE6F1'], N=256)

plt.matshow(some_matrix, cmap=cmap)

plt.show()

answered May 24, 2016 at 12:27

tmdavison

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2 Comments

Daneel Olivaw Over a year ago

Ah brilliant! That's exactly what I wanted, I was missing the .from_list, thank you!

bigZ Over a year ago

By far the quickest way I found to create gradient cmaps; thanks for sharing!

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