Adding and customising legends in Matplotlib

Topics Covered

1. What is the use of legend in Matplotlib?
2. How to add a legend in Matplotlib?
3. How to change the position of a legend in Matplotlib?
4. How to change the font size of a legend in Matplotlib?
5. How to add color to a legend in Matplotlib?
6. How to add a legend title in Matplotlib?

What is the use of legend in Matplotlib?

A legend helps you understand the different datasets plotted in your graph. They help provide meaning to various elements of the visualization.

How to add a legend in Matplotlib?

In Matplotlib, you can use the legend() function to create legends. Let's build a simple plot showing both sin and cosine functions.

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(0, 10, 1000)
fig, ax = plt.subplots()
ax.plot(x, np.sin(x), '-b', label='Sine')
ax.plot(x, np.cos(x), '--r', label='Cosine')
ax.axis('equal')
leg = ax.legend()
plt.show()

How to change the position of a legend in Matplotlib?

You can use the loc() parameter to change the position of a legend in a matplotlib plot.

import matplotlib.pyplot as plt
import numpy as np

fig, axs = plt.subplots(2, 2)
x = np.linspace(0, 10, 1000)
fig, axs = plt.subplots(2,2)

legend_loc = ['upper left', 'upper right', 'bottom left', 'bottom right']

axs[0,0].plot(x, np.sin(x), '-b', label='Sine')
axs[0,0].plot(x, np.cos(x), '--r', label='Cosine')
axs[0,0].axis('equal')
axs[0,0].legend(loc='upper left')

axs[0,1].plot(x, np.sin(x), '-b', label='Sine')
axs[0,1].plot(x, np.cos(x), '--r', label='Cosine')
axs[0,1].axis('equal')
axs[0,1].legend(loc='upper right')

axs[1,0].plot(x, np.sin(x), '-b', label='Sine')
axs[1,0].plot(x, np.cos(x), '--r', label='Cosine')
axs[1,0].axis('equal')
axs[1,0].legend(loc='lower left')

axs[1,1].plot(x, np.sin(x), '-b', label='Sine')
axs[1,1].plot(x, np.cos(x), '--r', label='Cosine')
axs[1,1].axis('equal')
axs[1,1].legend(loc='lower right')

plt.show()

How to change the font size of a legend in Matplotlib?

You can use the fontsize parameter to change the size of legend in Matplotlib. The accepted values are xx-small, x-small, small, medium, large, x-large and xx-large.

import matplotlib.pyplot as plt
import numpy as np

fig, axs = plt.subplots(2)
x = np.linspace(0, 10, 1000)
fig, axs = plt.subplots(2)

legend_loc = ['upper left', 'upper right', 'bottom left', 'bottom right']

axs[0].plot(x, np.sin(x), '-b', label='Sine')
axs[0].plot(x, np.cos(x), '--r', label='Cosine')
axs[0].axis('equal')
axs[0].legend(loc='upper left', fontsize='x-small')

axs[1].plot(x, np.sin(x), '-b', label='Sine')
axs[1].plot(x, np.cos(x), '--r', label='Cosine')
axs[1].axis('equal')
axs[1].legend(loc='upper right', fontsize='x-large')

plt.show()

How to add color to a legend in Matplotlib?

With certain plots, it might be useful to add colors to your legends. There are 3 parameters that you can use to specify colors to a legend.

labelcolor - used to  change the color of the text.

facecolor - used to change background color of the legend.

edgecolor - used to change the edge color of the legend.

import matplotlib.pyplot as plt
import numpy as np

fig, axs = plt.subplots(2)
x = np.linspace(0, 10, 1000)
fig, axs = plt.subplots(2)

legend_loc = ['upper left', 'upper right', 'bottom left', 'bottom right']

axs[0].plot(x, np.sin(x), '-b', label='Sine')
axs[0].plot(x, np.cos(x), '--r', label='Cosine')
axs[0].axis('equal')
axs[0].legend(loc='upper left', fontsize='x-small', labelcolor='white', facecolor='black',
           edgecolor='red')

axs[1].plot(x, np.sin(x), '-b', label='Sine')
axs[1].plot(x, np.cos(x), '--r', label='Cosine')
axs[1].axis('equal')
axs[1].legend(loc='upper right', fontsize='x-large', labelcolor='brown', facecolor='white',
           edgecolor='black')

plt.show()

How to add a legend title in Matplotlib?

To add a legend title in Matplotlib, use the title parameter. You can increase or decrease its size using the title_fontsize parameter.

import matplotlib.pyplot as plt
import numpy as np

fig, axs = plt.subplots(2)
x = np.linspace(0, 10, 1000)
fig, axs = plt.subplots(2)

legend_loc = ['upper left', 'upper right', 'bottom left', 'bottom right']

axs[0].plot(x, np.sin(x), '-b', label='Sine')
axs[0].plot(x, np.cos(x), '--r', label='Cosine')
axs[0].axis('equal')
axs[0].legend(loc='upper left', title = 'Functions', title_fontsize ='x-small')

axs[1].plot(x, np.sin(x), '-b', label='Sine')
axs[1].plot(x, np.cos(x), '--r', label='Cosine')
axs[1].axis('equal')
axs[1].legend(loc='upper right', title = 'Functions', title_fontsize ='x-large')

plt.show()

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