Join two colormaps in imshow

Question

I want to make a plot using imshow(), but with a little custom colormap. For positive values I want to use e.q. "gnuplot" colors and for neagtive "cool". Any idea how to do this?

Your problem looks similar to http://stackoverflow.com/questions/24997926/making-a-custom-colormap-using-matplotlib-in-python — Scott, May 06 '15 at 16:33
also see http://stackoverflow.com/questions/16834861/create-own-colormap-using-matplotlib-and-plot-color-scale -- note registering the colormap, which is maybe only mentioned in the comments. — cphlewis, May 06 '15 at 17:07

unutbu · Accepted Answer · 2015-05-06T19:50:35.560

You could use

cmap, norm = mcolors.from_levels_and_colors(levels, colors)

to construct a custom colormap.

For example,

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

data = np.linspace(-10,5,20).reshape(5,4)

cmap = {name:plt.get_cmap(name) for name in ('gnuplot', 'cool')}
N = 50

vmin, vmax = (np.nextafter(min(data.min(), -1), -np.inf), 
              np.nextafter(max(data.max(), 1), np.inf))              # 1

levels = np.concatenate([np.linspace(vmin, 0, N, endpoint=False),
                         np.linspace(0, vmax, N+1, endpoint=True)])  # 2
colors = np.concatenate([cmap[name](np.linspace(0, 1, N)) 
                         for name in ('cool', 'gnuplot')])           # 3

cmap, norm = mcolors.from_levels_and_colors(levels, colors)

plt.imshow(data, cmap=cmap,
           norm=norm, 
           interpolation='nearest')
bar = plt.colorbar()
plt.show()

yields

enter image description here

I wish the definition of vmin, vmax could be simpler. min(data.min(), -1) is used instead of data.min() in case data is positive. We need vmin to be negative for the custom colormap to be properly constructed. The np.nextafter nudges vmin down a little bit so the levels will encapsulate all the values in data. Without np.nextafter, imshow has a big white block when data equals vmax.
The colorbar levels go from vmin to 0 (exclusive), then 0 (inclusive) to vmax.
The colorbar colors are a concatenation of N colors sampled from cool, followed by N colors sampled from gnuplot. You can change N to control how many gradations of color from each underlying colormap you wish to have.

Join two colormaps in imshow

1 Answers1