Multiple legends in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of multiple legends with different labels

What will be the output of this code snippet that creates two legends for two different line plots?

Matplotlib

import matplotlib.pyplot as plt

x = [1, 2, 3]
y1 = [2, 4, 6]
y2 = [1, 3, 5]

line1, = plt.plot(x, y1, label='Line 1')
line2, = plt.plot(x, y2, label='Line 2')

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

plt.show()

A No legend appears because plt.legend() is called twice

B Only one legend appears with both 'Line 1' and 'Line 2' labels

C Only one legend appears with label 'Line 2' on upper right

D Two legends appear: 'Line 1' on upper left and 'Line 2' on upper right

💡 Hint

Think about how adding the first legend as an artist affects the second legend call.

The first legend is created for 'Line 1' and added explicitly to the axes as an artist. Then the second legend is created for 'Line 2' and placed separately. This results in two distinct legends shown on the plot.

🧠 Conceptual

intermediate

Why add_artist() is needed for multiple legends?

Why do we need to use add_artist() when adding multiple legends in matplotlib?

A Because matplotlib only supports one legend by default, add_artist() adds the first legend explicitly so the second can be added

B Because add_artist() automatically merges all legends into one

C Because add_artist() removes the first legend to add a new one

D Because add_artist() changes the legend font size

💡 Hint

Think about how matplotlib handles legends internally.

Matplotlib allows only one legend per axes by default. Using add_artist() adds the first legend as a fixed artist, freeing the axes to add another legend separately.

🔧 Debug

advanced

Identify the error in multiple legends code

What error will this code produce when trying to add two legends without add_artist()?

Matplotlib

import matplotlib.pyplot as plt

x = [1, 2, 3]
y1 = [2, 4, 6]
y2 = [1, 3, 5]

plt.plot(x, y1, label='Line 1')
plt.plot(x, y2, label='Line 2')

plt.legend(loc='upper left')
plt.legend(loc='upper right')

plt.show()

A No legend appears because second call overwrites the first

B Only one legend appears at upper right, no error

C SyntaxError due to multiple plt.legend() calls

D RuntimeError: Multiple legends not supported

💡 Hint

What happens when plt.legend() is called twice without add_artist()?

Calling plt.legend() twice replaces the first legend with the second. No error occurs, but only the last legend is visible.

❓ visualization

advanced

Visualize multiple legends with different styles

Which option correctly creates two legends with different colors and markers for two scatter plots?

Matplotlib

import matplotlib.pyplot as plt

x = [1, 2, 3]
y1 = [2, 4, 6]
y2 = [1, 3, 5]

scatter1 = plt.scatter(x, y1, color='red', marker='o', label='Red circles')
scatter2 = plt.scatter(x, y2, color='blue', marker='x', label='Blue crosses')

legend1 = plt.legend(handles=[scatter1], loc='upper left')
plt.gca().add_artist(legend1)
plt.legend(handles=[scatter2], loc='upper right')

plt.show()

A Only blue crosses legend appears on upper right

B One legend appears with both red circles and blue crosses combined

C Two legends appear: red circles on upper left, blue crosses on upper right

D No legends appear because scatter plots do not support legends

💡 Hint

Check how handles and add_artist() are used for multiple legends.

Each scatter plot is assigned a legend handle. The first legend is added as an artist, allowing the second legend to be added separately. This shows two distinct legends with different colors and markers.

🚀 Application

expert

Create multiple legends for grouped bar chart

You have a grouped bar chart with two groups: 'Men' and 'Women', each with bars for '2019' and '2020'. How do you create two legends: one for gender and one for year?

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

labels = ['A', 'B', 'C']
men_2019 = [5, 7, 6]
women_2019 = [6, 9, 5]
men_2020 = [7, 8, 7]
women_2020 = [8, 10, 6]

x = np.arange(len(labels))
width = 0.2

fig, ax = plt.subplots()

bar1 = ax.bar(x - width, men_2019, width, label='Men 2019', color='blue')
bar2 = ax.bar(x, women_2019, width, label='Women 2019', color='orange')
bar3 = ax.bar(x + width, men_2020, width, label='Men 2020', color='green')
bar4 = ax.bar(x + 2*width, women_2020, width, label='Women 2020', color='red')

# Your code to add two legends here

plt.show()

A Create legend1 with handles [bar1, bar2] labeled 'Men', 'Women'; add_artist(legend1); create legend2 with handles [bar1, bar3] labeled '2019', '2020'

B Create legend1 with handles [bar1, bar2, bar3, bar4] labeled 'Men', 'Women', '2019', '2020'; no add_artist() needed

C Create legend1 with handles [bar1, bar3] labeled 'Men', 'Women'; add_artist(legend1); create legend2 with handles [bar2, bar4] labeled '2019', '2020'

D Create legend1 with handles [bar1, bar2] labeled '2019', '2020'; add_artist(legend1); create legend2 with handles [bar3, bar4] labeled 'Men', 'Women'

💡 Hint

Group bars by gender and year, then create legends accordingly using add_artist() for the first legend.

Legend1 groups bars by gender using bar1 and bar2 handles. It is added as an artist to keep it fixed. Legend2 groups bars by year using bar1 and bar3 handles. This creates two separate legends for gender and year.

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