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.