Multiple legends in Matplotlib - Step-by-Step Execution

Concept Flow - Multiple legends

Create first plot elements

↓

Create first legend

↓

Create second plot elements

↓

Create second legend

↓

Display plot with both legends

First, plot data and create the first legend. Then plot more data and create a second legend. Finally, show the plot with both legends visible.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt

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

leg1 = plt.legend(handles=[line1], loc='upper left')
plt.gca().add_artist(leg1)
leg2 = plt.legend(handles=[line2], loc='upper right')

plt.show()

This code plots two lines and adds two separate legends for each line at different locations.

Execution Table

Step	Action	Plot Elements	Legend Created	Legend Location	Result
1	Plot line1	line1 plotted	None	None	Line 1 appears on plot
2	Plot line2	line1, line2 plotted	None	None	Line 2 appears on plot
3	Create legend for line1	line1, line2 plotted	leg1	upper left	First legend added
4	Add first legend to axes	line1, line2 plotted	leg1	upper left	First legend visible
5	Create legend for line2	line1, line2 plotted	leg1, leg2	upper left, upper right	Both legends visible
6	Show plot	line1, line2 plotted	leg1, leg2	upper left, upper right	Plot displayed with two legends

💡 Plot displayed with both legends visible at specified locations.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 5	Final
line1	None	Line1 object	Line1 object	Line1 object	Line1 object	Line1 object
line2	None	None	Line2 object	Line2 object	Line2 object	Line2 object
leg1	None	None	None	Legend1 object	Legend1 object	Legend1 object
leg2	None	None	None	None	Legend2 object	Legend2 object

Key Moments - 2 Insights

Why do we need to call add_artist() for the first legend?

Because creating the second legend replaces the first by default, add_artist() keeps the first legend visible as a separate artist (see step 4 in execution_table).

What happens if we don't assign legends to variables?

We can't add them separately or control their positions easily, so only one legend might appear (refer to steps 3 and 5).

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what is the legend location for leg1?

A lower left

B upper left

C upper right

D lower right

💡 Hint

Check the 'Legend Location' column at step 4 in execution_table.

At which step is the second legend (leg2) created?

A Step 5

B Step 3

C Step 2

D Step 6

💡 Hint

Look at the 'Action' column in execution_table for legend creation steps.

If we omit plt.gca().add_artist(leg1), what happens to the first legend?

A It moves to the bottom

B It still shows up normally

C It disappears when the second legend is added

D It overlaps with the second legend

💡 Hint

Refer to key_moments about add_artist() and step 4 in execution_table.

Concept Snapshot

Multiple legends in matplotlib:
- Plot lines and save their handles.
- Create first legend with plt.legend(handles=[...], loc=...).
- Use plt.gca().add_artist() to keep first legend.
- Create second legend similarly.
- Show plot with both legends visible.

Full Transcript

This example shows how to add multiple legends to a matplotlib plot. First, we plot two lines and save their references. Then, we create the first legend for the first line and add it to the axes using add_artist() to keep it visible. Next, we create the second legend for the second line. Finally, we display the plot with both legends visible at different locations. This method prevents the second legend from replacing the first. Variables like line1, line2, leg1, and leg2 track the plot elements and legends. Key points include the need for add_artist() to keep multiple legends and assigning legends to variables for control.

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