How to add legend below subplots in matplotlib?

问题:

I am trying to add a legend below a 3-column subplot figure.

I have tried the following:

fig, ax = plt.subplots(ncols=3)
ax[0].plot(data1)
ax[1].plot(data2)
ax[2].plot(data3)

ax_sub = plt.subplot(111)
box = ax_sub.get_position()
ax_sub.set_position([box.x0, box.y0 + box.height * 0.1,box.width, box.height * 0.9])
ax_sub.legend(['A', 'B', 'C'],loc='upper center', bbox_to_anchor=(0.5, -0.3),fancybox=False, shadow=False, ncol=3)
plt.show()

However, this creates just one empty frame. When I comment out the ax_sub part, my subplots show up nice (but without a legend...)...

Many thanks!

This is closely related to How to put the legend out of the plot

回答1:

The legend needs to know what it should show. By default it will take the labeled artist from the axes it is created in. Since here the axes ax_sub is empty, the legend will be empty as well.

The use of the ax_sub may anyways not make too much sense. We could instead use the middle axes (ax[1]) to place the legend. However, we still need all artists that should appear in the legend. For lines this is easy; one would supply a list of lines as handles to the handles argument.

import matplotlib.pyplot as plt
import numpy as np

data1,data2,data3 = np.random.randn(3,12)

fig, ax = plt.subplots(ncols=3)
l1, = ax[0].plot(data1)
l2, = ax[1].plot(data2)
l3, = ax[2].plot(data3)

fig.subplots_adjust(bottom=0.3, wspace=0.33)

ax[1].legend(handles = [l1,l2,l3] , labels=['A', 'B', 'C'],loc='upper center', 
             bbox_to_anchor=(0.5, -0.2),fancybox=False, shadow=False, ncol=3)
plt.show()