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.