Multiple legends in Matplotlib - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add the first legend to the plot.

Matplotlib

import matplotlib.pyplot as plt

plt.plot([1, 2, 3], label='Line 1')
plt.plot([3, 2, 1], label='Line 2')
plt.[1]()
plt.show()

Drag options to blanks, or click blank then click option'

Alegend

Btitle

Cxlabel

Dylabel

Attempts:

3 left

💡 Hint

Common Mistakes

Using title() instead of legend().

Forgetting to call any function to show the legend.

The legend() function adds a legend to the plot showing labels.

2fill in blank

medium

Complete the code to create a second legend for the scatter points.

Matplotlib

import matplotlib.pyplot as plt

line1, = plt.plot([1, 2, 3], label='Line 1')
line2, = plt.plot([3, 2, 1], label='Line 2')
scatter = plt.scatter([1, 2, 3], [3, 2, 1], label='Points')
plt.legend(handles=[line1, line2])
plt.[1](handles=[scatter], loc='upper right')
plt.show()

Drag options to blanks, or click blank then click option'

Aylabel

Btitle

Cxlabel

Dlegend

Attempts:

3 left

💡 Hint

Common Mistakes

Using title() or other functions instead of legend().

Not specifying the handles parameter for the second legend.

To add a second legend, call plt.legend() again with the new handles.

3fill in blank

hard

Fix the error in the code to correctly add two legends to the plot.

Matplotlib

import matplotlib.pyplot as plt

line1, = plt.plot([1, 2, 3], label='Line 1')
line2, = plt.plot([3, 2, 1], label='Line 2')
scatter = plt.scatter([1, 2, 3], [3, 2, 1], label='Points')
leg1 = plt.legend(handles=[line1, line2], loc='upper left')
plt.add_artist([1])
plt.legend(handles=[scatter], loc='upper right')
plt.show()

Drag options to blanks, or click blank then click option'

Aleg1

Bscatter

Cline1

Dline2

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to add the scatter object instead of the legend.

Not adding the first legend as an artist before the second legend.

When adding multiple legends, the first legend must be added as an artist using plt.gca().add_artist() or plt.add_artist() with the legend object.

4fill in blank

hard

Fill both blanks to create two legends and add the first legend as an artist.

Matplotlib

import matplotlib.pyplot as plt

line1, = plt.plot([1, 2, 3], label='Line 1')
line2, = plt.plot([3, 2, 1], label='Line 2')
scatter = plt.scatter([1, 2, 3], [3, 2, 1], label='Points')
leg1 = plt.[1](handles=[line1, line2], loc='upper left')
plt.[2](leg1)
plt.legend(handles=[scatter], loc='upper right')
plt.show()

Drag options to blanks, or click blank then click option'

Alegend

Badd_artist

Ctitle

Dxlabel

Attempts:

3 left

💡 Hint

Common Mistakes

Using title() or xlabel() instead of add_artist().

Not adding the first legend as an artist before the second legend.

First, create the legend with plt.legend(). Then add it as an artist with plt.add_artist() before adding the second legend.

5fill in blank

hard

Fill all three blanks to create two legends with different locations and add the first legend as an artist.

Matplotlib

import matplotlib.pyplot as plt

line1, = plt.plot([1, 2, 3], label='Line 1')
line2, = plt.plot([3, 2, 1], label='Line 2')
scatter = plt.scatter([1, 2, 3], [3, 2, 1], label='Points')
leg1 = plt.[1](handles=[line1, line2], loc=[2])
plt.[3](leg1)
plt.legend(handles=[scatter], loc='upper right')
plt.show()

Drag options to blanks, or click blank then click option'

Alegend

B'upper left'

Cadd_artist

D'lower right'

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong location strings or forgetting quotes.

Not adding the first legend as an artist.

Create the first legend with plt.legend() at 'upper left'. Then add it as an artist with plt.add_artist() before adding the second legend at 'upper right'.

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