How to Create a Single Legend for Multiple Matplotlib Subplots?

Creating a Single Legend for Multiple Matplotlib Subplots

When plotting similar information across multiple subplots using Matplotlib, it can be beneficial to create a single legend that applies to all subplots. This simplifies the interpretation of the data by providing a consistent reference for the lines in each subplot.

To achieve this, utilize the get_legend_handles_labels() function on the last axis or by calling plt.gca().get_legend_handles_labels() if using the pyplot interface. These functions collect the necessary legend handles and labels from the label= arguments.

To create the single legend, call fig.legend(handles, labels, loc='upper center'), where fig is the figure containing the subplots and loc specifies the legend's location.

For example, if you have a 3x3 grid of subplots with identical lines, the following code would add a single legend above all subplots:

import matplotlib.pyplot as plt
import numpy as np

# Generate data for the subplots
data = np.random.rand(9)

# Create the subplots
fig, axes = plt.subplots(3, 3)

# Plot the data on each subplot
for ax, datum in zip(axes.flatten(), data):
    ax.plot(datum)

# Get the legend handles and labels
handles, labels = plt.gca().get_legend_handles_labels()

# Create the single legend
plt.legend(handles, labels, loc='upper center')

plt.show()

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