How to add legend below subplots in matplotlib?

Question

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

What do you want to show in the legend? And where in the code do you specify that? — ImportanceOfBeingErnest, Sep 17 '17 at 18:44
In this example it is the `['A', 'B', 'C']` in `ax_sub.legend()`. — knut_h, Sep 17 '17 at 18:52
I suppose the problem is that adding a `plt.subplot` does not work in a figure that is comprised of `plt.subplots` — knut_h, Sep 17 '17 at 18:53
No, that is not really a problem. My problem is to understand what you are trying to achieve here. Should the legend consist of a list of letters? — ImportanceOfBeingErnest, Sep 17 '17 at 18:59
Since I provided an [answer](https://stackoverflow.com/a/43439132/4124317) to the linked question ("How to put the legend out of the plot") as well, I would be interested in how far it did not help you. Any suggestion on improving it? — ImportanceOfBeingErnest, Sep 17 '17 at 20:26
really great answer thanks! Just did not have a chance to respond last night anymore... — knut_h, Sep 18 '17 at 11:18

score 13 · Accepted Answer · answered Sep 17 '17 at 19:08

13

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()

answered Sep 17 '17 at 19:08

ImportanceOfBeingErnest

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great! Do you by any chance know how to change label(tab-)spacing? I only know `labelspacing = XXX`, which only changes the linespacing of labels. – knut_h Sep 18 '17 at 12:04
Do you mean [`columnspacing`](https://matplotlib.org/api/pyplot_api.html#matplotlib.pyplot.legend)? – ImportanceOfBeingErnest Sep 18 '17 at 16:49
exactly! so the spacing between -A, -B, and -C in your example. – knut_h Sep 29 '17 at 11:21
Does this work for even number subplots? – Dawei Wang Mar 05 '22 at 08:25

How to add legend below subplots in matplotlib?

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