Legend outside the plot in Matplotlib - Step-by-Step Execution

Concept Flow - Legend outside the plot

Create plot with data

↓

Add legend inside plot

↓

Adjust legend position

↓

Place legend outside plot area

↓

Display final plot

This flow shows how to create a plot, add a legend, then move the legend outside the plot area for better visibility.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt

plt.plot([1, 2, 3], label='Line 1')
plt.plot([3, 2, 1], label='Line 2')
plt.legend(loc='center left', bbox_to_anchor=(1, 0.5))
plt.show()

This code plots two lines and places the legend outside the plot on the right side.

Execution Table

Step	Action	Legend Location	Legend Coordinates	Plot Displayed
1	Plot line 1 with label 'Line 1'	None	None	No
2	Plot line 2 with label 'Line 2'	None	None	No
3	Add legend with default location	Best (inside plot)	(default)	No
4	Change legend location to 'center left' outside plot	center left	(1, 0.5)	No
5	Show plot with legend outside	center left	(1, 0.5)	Yes

💡 Plot displayed with legend outside on the right at coordinates (1, 0.5)

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
legend location	None	None	None	best (inside)	center left	center left
legend bbox_to_anchor	None	None	None	default	(1, 0.5)	(1, 0.5)
plot displayed	False	False	False	False	False	True

Key Moments - 3 Insights

Why does the legend appear inside the plot before setting bbox_to_anchor?

By default, plt.legend() places the legend inside the plot at the 'best' location. This is shown in execution_table step 3 where the legend location is 'best (inside)'.

What does bbox_to_anchor=(1, 0.5) do to the legend position?

bbox_to_anchor sets the anchor point for the legend box. (1, 0.5) places it at the right center outside the plot area, as seen in step 4 and 5.

Why is the plot not displayed until plt.show() is called?

Matplotlib builds the plot step-by-step but only renders it on screen when plt.show() is called, as indicated by 'plot displayed' changing to True at step 5.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, where is the legend positioned?

A Outside the plot on the right center

B Inside the plot at the best location

C At the top left corner inside the plot

D At the bottom center inside the plot

💡 Hint

Check the 'Legend Location' and 'Legend Coordinates' columns at step 4 in execution_table

According to variable_tracker, when does 'plot displayed' become True?

A After Step 3

B After Step 5

C After Step 4

D At the start

💡 Hint

Look at the 'plot displayed' row in variable_tracker and see when it changes to True

If bbox_to_anchor was set to (0, 0), where would the legend appear relative to the plot?

A Inside the plot at the center

B Outside the plot on the top right

C Outside the plot on the bottom left

D Inside the plot at the top left

💡 Hint

bbox_to_anchor sets the anchor point; (0,0) is bottom left outside the plot area

Concept Snapshot

plt.legend() adds a legend to a plot.
Use loc to set legend position.
bbox_to_anchor moves legend outside plot.
Example: loc='center left', bbox_to_anchor=(1, 0.5) places legend right outside.
Call plt.show() to display the plot with legend.

Full Transcript

This example shows how to place a legend outside a matplotlib plot. First, two lines are plotted with labels. By default, the legend appears inside the plot at the best location. Then, using plt.legend with loc='center left' and bbox_to_anchor=(1, 0.5), the legend moves outside the plot on the right center. The plot is displayed only after plt.show() is called. Variables like legend location and plot display status change step-by-step as shown in the tables.

Practice

1. What is the main purpose of placing a legend outside the plot in matplotlib?

easy

A. To keep the data area clear and improve readability

B. To make the plot smaller

C. To hide the legend from view

D. To change the plot colors automatically

Solution

1. Step 1: Understand legend placement

   Legends inside the plot can cover data points, making the plot hard to read.

2. Step 2: Effect of placing legend outside

   Placing the legend outside keeps the data visible and the plot clear.

3. Final Answer:

   To keep the data area clear and improve readability -> Option A

4. Quick Check:

   Legend outside = clear data area [OK]

Hint: Legend outside keeps plot data visible and neat [OK]

Common Mistakes:

• Thinking legend outside hides it
• Assuming it changes plot size automatically
• Believing it changes plot colors

2. Which of the following is the correct way to place a legend outside the plot on the right using bbox_to_anchor and loc?

easy

A. plt.legend(loc='upper left', bbox_to_anchor=(-1, 1))

B. plt.legend(loc='center', bbox_to_anchor=(0, 0))

C. plt.legend(loc='lower left', bbox_to_anchor=(0.5, 0.5))

D. plt.legend(loc='upper right', bbox_to_anchor=(1.05, 1))

Solution

1. Step 1: Understand bbox_to_anchor coordinates

   The tuple (1.05, 1) places the legend just outside the right edge at the top.

2. Step 2: Match loc with bbox_to_anchor

   Using loc='upper right' anchors the legend's upper right corner to the point (1.05, 1).

3. Final Answer:

   plt.legend(loc='upper right', bbox_to_anchor=(1.05, 1)) -> Option D

4. Quick Check:

   Right outside = bbox_to_anchor > 1 on x [OK]

Hint: Use bbox_to_anchor >1 on x with loc='upper right' for right outside [OK]

Common Mistakes:

• Using bbox_to_anchor inside plot area (less than 1)
• Mismatching loc and bbox_to_anchor
• Using negative bbox_to_anchor values incorrectly

3. What will be the effect of the following code snippet?

import matplotlib.pyplot as plt
plt.plot([1, 2, 3], label='Line')
plt.legend(loc='center left', bbox_to_anchor=(1, 0.5))
plt.show()

medium

A. Legend appears outside the plot on the right center

B. Legend appears inside the plot at center left

C. Legend appears below the plot

D. Code will raise an error

Solution

1. Step 1: Analyze bbox_to_anchor coordinates

   The point (1, 0.5) is at the right edge horizontally and center vertically.

2. Step 2: Understand loc='center left'

   The legend's center left side is anchored at (1, 0.5), placing it just outside the plot on the right center.

3. Final Answer:

   Legend appears outside the plot on the right center -> Option A

4. Quick Check:

   loc='center left' + bbox_to_anchor=(1,0.5) = right outside center [OK]

Hint: loc='center left' with bbox_to_anchor=(1,0.5) puts legend right outside center [OK]

Common Mistakes:

• Thinking legend is inside plot
• Confusing loc and bbox_to_anchor roles
• Expecting legend below plot

4. Identify the error in this code that tries to place the legend outside the plot on the right:

import matplotlib.pyplot as plt
plt.plot([1, 2, 3], label='Data')
plt.legend(loc='outer_right', bbox_to_anchor=(1.05, 1))
plt.show()

medium

A. Missing plt.tight_layout() causes overlap

B. bbox_to_anchor coordinates are out of range

C. Invalid loc value 'outer_right' causes an error

D. No label provided for the plot

Solution

1. Step 1: Check loc parameter validity

   Matplotlib does not support 'outer_right' as a valid loc string; valid locs include 'upper right', 'center left', etc.

2. Step 2: Effect of invalid loc

   Using 'outer_right' causes a ValueError and the legend will not display.

3. Final Answer:

   Invalid loc value 'outer_right' causes an error -> Option C

4. Quick Check:

   loc must be valid string like 'upper right' [OK]

Hint: Use only valid loc strings like 'upper right', not 'outer_right' [OK]

Common Mistakes:

• Using invalid loc strings
• Ignoring need for plt.tight_layout()
• Forgetting to add label to plot

5. You want to place a legend outside the plot on the right without overlapping the plot or cutting off the legend text. Which combination of commands is best to achieve this?

hard

A. plt.legend(loc='upper right', bbox_to_anchor=(0.5, 0.5)); plt.show()

B. plt.legend(loc='center left', bbox_to_anchor=(1, 0.5)); plt.tight_layout()

C. plt.legend(loc='lower left', bbox_to_anchor=(-0.5, 0)); plt.tight_layout()

D. plt.legend(loc='center', bbox_to_anchor=(1.5, 1.5)); plt.show()

Solution

1. Step 1: Choose correct legend position

   loc='center left' with bbox_to_anchor=(1, 0.5) places the legend just outside the right center of the plot.

2. Step 2: Use plt.tight_layout() to avoid overlap

   Calling plt.tight_layout() adjusts the plot layout to prevent the legend from overlapping or being cut off.

3. Final Answer:

   plt.legend(loc='center left', bbox_to_anchor=(1, 0.5)); plt.tight_layout() -> Option B

4. Quick Check:

   Right outside + tight_layout = no overlap [OK]

Hint: Use bbox_to_anchor outside + plt.tight_layout() to avoid overlap [OK]

Common Mistakes:

• Not using plt.tight_layout() causing overlap
• Placing legend inside plot by wrong bbox_to_anchor
• Using bbox_to_anchor with negative values incorrectly