Creating a Colormap Legend in Matplotlib

Question

I am using imshow() in matplotlib like so:

import numpy as np
import matplotlib.pyplot as plt
mat = '''SOME MATRIX'''
plt.imshow(mat, origin="lower", cmap='gray', interpolation='nearest')
plt.show()

How do I add a legend showing the numeric value for the different shades of gray. Sadly, my googling has not uncovered an answer 🙁

Thank you in advance for the help.

Vince

Asked By: Vince

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Source

Answer 1

As usual, I figure it out right after I ask it ;). For posterity, here’s my stab at it:

m = np.zeros((1,20))
for i in range(20):
    m[0,i] = (i*5)/100.0
print m
plt.imshow(m, cmap='gray', aspect=2)
plt.yticks(np.arange(0))
plt.xticks(np.arange(0,25,5), [0,25,50,75,100])
plt.show()

I’m sure there exists a more elegant solution.

Vince

Answered By: Vince

Answer 2

There’s a builtin colorbar() function in pyplot. Here’s an example using subplots:

fig = plt.figure()
ax = fig.add_subplot(1,1,1)
plot = ax.pcolor(data)
fig.colorbar(plot)

Answered By: Vicki Laidler

Answer 3

Simple, just plt.colorbar():

import numpy as np
import matplotlib.pyplot as plt
mat = np.random.random((10,10))
plt.imshow(mat, origin="lower", cmap='gray', interpolation='nearest')
plt.colorbar()
plt.show()

Answered By: wordsforthewise

Creating a Colormap Legend in Matplotlib

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