matplotlib.pyplot.imshow, issue with plt.legend()

Question

I have some pictures that I display using the function imshow of the matplotlib.pyplot (alias plt) library :

# mat is the matrix defining the image
plt.imshow(mat.reshape((16,16)),interpolation="nearest",cmap=cm.binary)
plt.legend("bla bla")

When I execute this, I get an image with a little empty square at the top right and I can't see the string I put in plt.legend. I tried to add label="bla bla" in imshow and then use plt.legend() with no arguments in it. In this case, even the little square which should normally display the label disappear.

Answer 1

imshow doesn't have parts that one would normally explain with a legend. If a colorbar or title don't work, you can add an annotation (as altered from matplotlib gallery):

import matplotlib.pyplot as plt
import numpy.random as nr

M = nr.random((20,20))
ax = plt.imshow(M).get_axes()

ax.annotate('anomaly', xy=(4., 1),  xycoords='data',
                xytext=(50, -120), textcoords='offset points',
                bbox=dict(boxstyle="round", fc="0.8"),
                arrowprops=dict(arrowstyle="->",
                                shrinkA=0, shrinkB=10,
                                connectionstyle="angle,angleA=0,angleB=90,rad=10"),
                )

plt.show()