python - Custom colour maps Matplotlib, make one value a prescribed colour

Question

I have an array in python, using matplotlib, with floats ranging between 0 and 1.

I am displaying this array with imshow, I am trying to create a custom cmap, which is identical to Greens, however when a cell becomes 0 I would like to be able to map that value to red, and leave the rest of he spectrum unchanged.

If anyone more familiar with matplotlib would be able to help me I would greatly appreciate it!

For instance how would I edit this script so that the zero value in the matrix showed as red?

import numpy as np

from matplotlib import pyplot as plt

import matplotlib

x = np.array([[0,1,2],[3,4,5],[6,7,8]])

fig = plt.figure()

cmap_custom = matplotlib.cm.Greens

plt.imshow( x, interpolation='nearest' ,cmap = cmap_custom)

plt.colorbar()

plt.show()

Answer 1

The colormaps in matplotlib allow you to set special colors for values that are outside of the defined range. In your case specify the color for values below the defined range with cmap_custom.set_under('r'). Then you also need to specify the lower end of the range: vmin=0.01 (just some value > 0). Finally create the colorbar with plt.colorbar(extend='min').

import numpy as np
from matplotlib import pyplot as plt
import matplotlib

x = np.array([[0,1,2],[3,4,5],[6,7,8]])

fig = plt.figure()
cmap_custom = matplotlib.cm.Greens
cmap_custom.set_under('r')
plt.imshow( x, interpolation='nearest' ,cmap = cmap_custom, vmin=0.01)
plt.colorbar(extend='min')
plt.show()