Legend outside the plot in Matplotlib

Introduction

Putting the legend outside the plot helps keep the plot area clear and easy to read. It avoids covering important data points.

When the legend has many items and takes too much space inside the plot.

When data points are dense and the legend overlaps them.

When you want a cleaner look with more space for the data visualization.

When preparing plots for presentations or reports where clarity is important.

Syntax

Matplotlib

plt.legend(loc='best', bbox_to_anchor=(x, y), borderaxespad=pad)

loc sets the anchor point of the legend box.

bbox_to_anchor moves the legend box outside the plot by specifying coordinates.

Examples

Places the legend outside the plot on the upper right side.

Matplotlib

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

Places the legend centered vertically on the right side outside the plot.

Matplotlib

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

Places the legend outside the plot at the bottom right corner.

Matplotlib

plt.legend(loc='lower left', bbox_to_anchor=(1, 0))

Sample Program

This code plots two lines and places the legend outside the plot area on the right side, centered vertically. The bbox_to_anchor moves the legend box outside the plot, and tight_layout() adjusts the plot to fit everything nicely.

Matplotlib

import matplotlib.pyplot as plt

x = [1, 2, 3, 4]
y1 = [10, 20, 25, 30]
y2 = [15, 18, 22, 27]

plt.plot(x, y1, label='Line 1')
plt.plot(x, y2, label='Line 2')

# Put legend outside the plot on the right
plt.legend(loc='center left', bbox_to_anchor=(1, 0.5))

plt.title('Plot with Legend Outside')
plt.xlabel('X axis')
plt.ylabel('Y axis')

plt.tight_layout()  # Adjust layout to make room for legend
plt.show()

Important Notes

Use plt.tight_layout() or adjust figure size to prevent clipping of the legend.

You can change bbox_to_anchor coordinates to move the legend to different outside positions.

Summary

Legends can be placed outside the plot to keep the plot area clear.

Use loc and bbox_to_anchor together to position the legend outside.

Adjust layout with tight_layout() to avoid overlap or clipping.

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