How to change colorbar's color (in some particular value interval)?

Question

In matplotlib, I would like to change colorbar's color in some particular value interval. For example, I would like to change the seismic colorbar, to let the values between -0.5 and 0.5 turn white, how can I do this?

thank you very much

Answer 1

You basically need to create your own colormap that has the particular features you want. Of course it is possible to make use of existing colormaps when doing so.

Colormaps are always ranged between 0 and 1. This range will then be mapped to the data interval. So in order to create whites between -0.5 and 0.5 we need to know the range of data - let's say data goes from -1 to 1. We can then decide to have the lower (blues) part of the seismic map go from -1 to -0.5, then have white between -0.5 and +0.5 and finally the upper part of the seismic map (reds) from 0.5 to 1. In the language of a colormap this corresponds to the ranges [0,0.25], [0.25, 0.75] and [0.75,1]. We can then create a list, with the first and last 25% percent being the colors of the seismic map and the middle 50% white.

This list can be used to create a colormap, using matplotlib.colors.LinearSegmentedColormap.from_list("colormapname", listofcolors).

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

n=50
x = 0.5
lower = plt.cm.seismic(np.linspace(0, x, n))
white = plt.cm.seismic(np.ones(100)*0.5)
upper = plt.cm.seismic(np.linspace(1-x, 1, n))
colors = np.vstack((lower, white, upper))
tmap = matplotlib.colors.LinearSegmentedColormap.from_list('terrain_map_white', colors)

x = np.linspace(0,10)
X,Y = np.meshgrid(x,x)
z = np.sin(X) * np.cos(Y*0.4)

fig, ax = plt.subplots()
im = ax.imshow(z, cmap=tmap)    
plt.colorbar(im)

plt.show() 

For more general cases, you may need a color normalization (using matplotlib.colors.Normalize). See e.g. this example, where a certain color in the colormap is always fixed at a data value of 0, independent of the data range.