Multiple legends in Matplotlib

Introduction

Sometimes you want to explain different groups of lines or points separately in one plot. Multiple legends help you do that clearly.

You have a plot with lines of different colors and styles representing different categories.

You want to show one legend for data groups and another for marker shapes.

You need to explain multiple sets of data on the same graph separately.

You want to keep the plot clean by splitting legend information into parts.

Syntax

Matplotlib

legend1 = plt.legend(handles=group1_handles, loc='upper left')
plt.gca().add_artist(legend1)
plt.legend(handles=group2_handles, loc='upper right')

You create the first legend and add it to the plot as an artist.

Then you create the second legend normally. This way both show up.

Examples

This example shows two lines with separate legends on opposite corners.

Matplotlib

import matplotlib.pyplot as plt

line1, = plt.plot([1, 2, 3], label='Group A')
line2, = plt.plot([3, 2, 1], label='Group B')

legend1 = plt.legend(handles=[line1], loc='upper left')
plt.gca().add_artist(legend1)
plt.legend(handles=[line2], loc='upper right')
plt.show()

This example shows how to add one legend for lines and another for scatter points with titles.

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(0, 10, 100)
line1, = plt.plot(x, np.sin(x), 'r-', label='Sine wave')
line2, = plt.plot(x, np.cos(x), 'b--', label='Cosine wave')

scatter = plt.scatter([2, 4, 6], [0, 0, 0], c='g', label='Points')

legend1 = plt.legend(handles=[line1, line2], title='Lines', loc='upper left')
plt.gca().add_artist(legend1)
plt.legend(handles=[scatter], title='Markers', loc='lower right')
plt.show()

Sample Program

This program plots two decay curves and some sample points. It adds two legends: one for the lines and one for the points, placed in different corners for clarity.

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

# Create data
x = np.linspace(0, 5, 100)
y1 = np.exp(-x)
y2 = np.exp(-0.5 * x)

# Plot two lines
line1, = plt.plot(x, y1, 'b-', label='Fast decay')
line2, = plt.plot(x, y2, 'r--', label='Slow decay')

# Plot some points
points = plt.scatter([1, 2, 3], [0.8, 0.6, 0.4], c='green', label='Samples')

# First legend for lines
legend1 = plt.legend(handles=[line1, line2], title='Decay types', loc='upper right')
plt.gca().add_artist(legend1)

# Second legend for points
plt.legend(handles=[points], title='Data points', loc='lower left')

plt.xlabel('Time')
plt.ylabel('Value')
plt.title('Multiple Legends Example')
plt.show()

Important Notes

Always add the first legend as an artist before adding the second one.

Use different locations to avoid legends overlapping.

You can customize legend titles and styles for better clarity.

Summary

Multiple legends help explain different data groups clearly in one plot.

Create the first legend and add it as an artist, then add the second normally.

Place legends in different locations to keep the plot readable.

Practice

1. What is the main reason to use multiple legends in a matplotlib plot?

easy

A. To increase the plot size automatically

B. To change the color of the plot lines

C. To label different groups or data sets separately on the same plot

D. To save the plot as an image file

Solution

1. Step 1: Understand the purpose of legends

   Legends help identify what each line or marker in a plot represents.

2. Step 2: Recognize why multiple legends are needed

   When a plot has multiple groups or categories, separate legends make it easier to understand each group distinctly.

3. Final Answer:

   To label different groups or data sets separately on the same plot -> Option C

4. Quick Check:

   Multiple legends = separate group labels [OK]

Hint: Multiple legends label different groups clearly [OK]

Common Mistakes:

• Thinking legends change plot colors
• Confusing legends with saving files
• Assuming legends resize plots

2. Which matplotlib method is used to keep the first legend visible when adding a second legend?

easy

A. ax.add_artist()

B. plt.show()

C. plt.legend()

D. ax.set_title()

Solution

1. Step 1: Identify how legends are added

   Legends are usually added with plt.legend() or ax.legend().

2. Step 2: Understand how to keep multiple legends

   To keep the first legend visible after adding a second, you must add the first legend as an artist using ax.add_artist().

3. Final Answer:

   ax.add_artist() -> Option A

4. Quick Check:

   Keep first legend visible = add_artist() [OK]

Hint: Use add_artist() to keep first legend visible [OK]

Common Mistakes:

• Using plt.legend() twice without add_artist()
• Calling plt.show() to fix legends
• Trying to set title to fix legends

3. What will the following code display?

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
line1, = ax.plot([1, 2, 3], label='Line 1')
line2, = ax.plot([3, 2, 1], label='Line 2')
leg1 = ax.legend(handles=[line1], loc='upper left')
ax.add_artist(leg1)
ax.legend(handles=[line2], loc='upper right')
plt.show()

medium

A. Two legends: 'Line 1' at upper left and 'Line 2' at upper right

B. Only one legend showing 'Line 2' at upper right

C. Only one legend showing 'Line 1' at upper left

D. No legends displayed

Solution

1. Step 1: Analyze legend creation

   The first legend is created for 'Line 1' and added as an artist to keep it visible.

2. Step 2: Analyze second legend addition

   The second legend is created for 'Line 2' and placed at upper right. Both legends appear separately.

3. Final Answer:

   Two legends: 'Line 1' at upper left and 'Line 2' at upper right -> Option A

4. Quick Check:

   add_artist() + second legend = two legends [OK]

Hint: add_artist() keeps first legend when adding second [OK]

Common Mistakes:

• Expecting only one legend to show
• Forgetting to add_artist() for first legend
• Placing both legends at same location

4. Identify the error in this code that tries to add two legends:

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
line1, = ax.plot([1, 2, 3], label='A')
line2, = ax.plot([3, 2, 1], label='B')
ax.legend(handles=[line1], loc='upper left')
ax.legend(handles=[line2], loc='upper right')
plt.show()

medium

A. Syntax error due to missing parentheses

B. Second legend call overwrites the first, so only one legend shows

C. Plot lines are not labeled correctly

D. The plot will not display because plt.show() is missing

Solution

1. Step 1: Understand legend behavior

   Calling ax.legend() twice replaces the previous legend each time.

2. Step 2: Identify why only one legend appears

   Without using ax.add_artist(), the first legend is removed when the second is added.

3. Final Answer:

   Second legend call overwrites the first, so only one legend shows -> Option B

4. Quick Check:

   Multiple legend calls overwrite without add_artist() [OK]

Hint: Use add_artist() to keep multiple legends visible [OK]

Common Mistakes:

• Assuming multiple legend() calls stack automatically
• Thinking missing plt.show() causes no plot
• Believing labels cause syntax errors

5. You want to plot three data groups with separate legends placed at 'upper left', 'upper right', and 'lower left' without overlap. Which approach correctly achieves this?

hard

A. Plot all lines without legends and add a single legend manually

B. Call ax.legend() three times without add_artist(), changing loc each time

C. Use plt.legend() once with all labels combined

D. Create first legend, add it with ax.add_artist(), then create second and third legends with different loc values

Solution

1. Step 1: Understand legend placement

   Legends placed at different loc positions avoid overlap visually.

2. Step 2: Use add_artist() to keep multiple legends

   Adding the first legend as an artist keeps it visible when adding others.

3. Step 3: Create all three legends with distinct locations

   Creating legends separately with loc set to 'upper left', 'upper right', and 'lower left' places them without overlap.

4. Final Answer:

   Create first legend, add it with ax.add_artist(), then create second and third legends with different loc values -> Option D

5. Quick Check:

   add_artist() + distinct loc = multiple legends no overlap [OK]

Hint: Add first legend as artist, set loc differently for others [OK]

Common Mistakes:

• Calling legend() multiple times without add_artist()
• Placing all legends at same location causing overlap
• Using single legend for all groups losing clarity