Drawing multiple legends on the same axes in Matplotlib

To draw multiple legends on the same axes in Matplotlib, we can create separate legends for different groups of lines and position them at different locations on the plot.

Steps to Create Multiple Legends

• Set the figure size and adjust the padding between and around the subplots
• Plot lines using different labels, linewidth and linestyle
• Create the first legend and place it at the upper-right location
• Add the first legend as an artist to the current axis using add_artist()
• Create the second legend and place it at the lower-right location
• Display the figure using show() method

Example

Here's how to create multiple legends on the same plot ?

import matplotlib.pyplot as plt

# Set figure size and layout
plt.rcParams["figure.figsize"] = [7.50, 3.50]
plt.rcParams["figure.autolayout"] = True

# Plot two lines with different styles
line1, = plt.plot([1, 2, 3], label="Line 1", linestyle='--')
line2, = plt.plot([3, 2, 1], label="Line 2", linewidth=4)

# Create first legend for line1
first_legend = plt.legend(handles=[line1], loc='upper right')

# Add first legend as an artist to preserve it
plt.gca().add_artist(first_legend)

# Create second legend for line2
plt.legend(handles=[line2], loc='lower right')

plt.show()

How It Works

The key to multiple legends is using add_artist() method. When you call plt.legend() a second time, Matplotlib replaces the first legend unless you explicitly add it as an artist to the axes.

Alternative Approach with Custom Positioning

You can also specify exact coordinates for legend placement ?

import matplotlib.pyplot as plt

plt.figure(figsize=(8, 6))

# Plot multiple lines
line1, = plt.plot([1, 2, 3, 4], [1, 4, 2, 3], 'r-', label='Red Line')
line2, = plt.plot([1, 2, 3, 4], [2, 3, 4, 1], 'b--', label='Blue Line')
line3, = plt.plot([1, 2, 3, 4], [3, 1, 4, 2], 'g:', label='Green Line')

# First legend - top right
legend1 = plt.legend(handles=[line1], loc='upper right')
plt.gca().add_artist(legend1)

# Second legend - bottom left using coordinates
legend2 = plt.legend(handles=[line2, line3], bbox_to_anchor=(0.02, 0.02), loc='lower left')

plt.title('Multiple Legends Example')
plt.xlabel('X-axis')
plt.ylabel('Y-axis')
plt.show()

Key Points

• add_artist() is essential to preserve the first legend when adding a second one
• Each legend can contain different line handles and be positioned independently
• Use bbox_to_anchor for precise positioning with coordinates
• The loc parameter works in conjunction with bbox_to_anchor for fine-tuned placement

Conclusion

Multiple legends in Matplotlib require using add_artist() to preserve earlier legends. This technique allows you to group different plot elements logically and position legends wherever needed on your plot.