Legend placement options in Matplotlib - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of a legend in a matplotlib plot?

A legend explains what each color, line, or marker in the plot represents, making the plot easier to understand.

beginner

Name three common locations where a legend can be placed in a matplotlib plot.

Common legend locations include 'upper right', 'upper left', and 'lower center'.

intermediate

How do you place a legend outside the plot area in matplotlib?

Use the 'bbox_to_anchor' parameter with 'loc' in plt.legend() to position the legend outside the plot area.

beginner

What does the 'loc' parameter control in matplotlib's legend function?

The 'loc' parameter controls the anchor point of the legend box relative to the plot or the bounding box.

intermediate

Explain the difference between 'best' and 'center left' legend locations in matplotlib.

'best' lets matplotlib automatically place the legend where it overlaps the least with the plot data, while 'center left' places the legend centered vertically on the left side of the plot.

Which parameter in plt.legend() sets the legend location?

A place

B position

C loc

D location

The 'loc' parameter is used to specify the legend location in matplotlib.

What does setting loc='best' do for the legend placement?

A Automatically finds the best location to avoid overlapping data

B Centers legend at the bottom

C Places legend outside the plot

D Places legend at the top right corner

'best' lets matplotlib choose the location with the least overlap with plot elements.

How can you place the legend outside the plot area on the right side?

A loc='right'

B loc='outside'

C bbox_to_anchor=(0, 0) with loc='upper left'

D bbox_to_anchor=(1, 0.5) with loc='center left'

Using bbox_to_anchor with loc='center left' places the legend outside on the right.

Which of these is NOT a valid loc value for legend placement?

A middle left

B lower center

C upper right

D best

'middle left' is not a valid loc value; the correct term is 'center left'.

What does bbox_to_anchor control in legend placement?

A The font size of the legend

B The exact position of the legend box

C The color of the legend background

D The number of legend columns

bbox_to_anchor sets the precise position of the legend box relative to the axes.

Describe how to place a legend outside the plot area on the right side using matplotlib.

Think about combining loc with bbox_to_anchor to move the legend.

List and explain at least three different legend placement options available in matplotlib.

Consider both fixed positions and automatic placement.

Practice

1. What does the loc parameter control in a matplotlib legend?

easy

A. The type of plot displayed

B. The color of the legend box

C. The font size of the legend text

D. The position of the legend inside or outside the plot area

Solution

1. Step 1: Understand the role of loc in legend placement

   The loc parameter specifies where the legend appears relative to the plot, such as 'upper right' or 'lower left'.

2. Step 2: Differentiate from other legend properties

   Other properties like color or font size are controlled by different parameters, not loc.

3. Final Answer:

   The position of the legend inside or outside the plot area -> Option D

4. Quick Check:

   Legend position = loc [OK]

Hint: Remember: loc sets legend position, not style [OK]

Common Mistakes:

• Confusing loc with color or font size
• Thinking loc controls plot type
• Assuming loc changes data points

2. Which of the following is the correct syntax to place a legend at the lower center outside the plot using loc and bbox_to_anchor?

easy

A. plt.legend(loc='lower center', bbox_to_anchor=(0.5, -0.1))

B. plt.legend(loc='center lower', bbox_to_anchor=(0.5, -0.1))

C. plt.legend(location='lower center', bbox_to_anchor=(0.5, -0.1))

D. plt.legend(loc='lower center', bbox_to_anchor=[0.5, -0.1])

Solution

1. Step 1: Check correct parameter names and values

   The parameter for legend position is loc, and the value 'lower center' is valid. The bbox_to_anchor expects a tuple for coordinates.

2. Step 2: Identify syntax errors in options

   plt.legend(loc='center lower', bbox_to_anchor=(0.5, -0.1)) uses 'center lower' which is invalid. plt.legend(location='lower center', bbox_to_anchor=(0.5, -0.1)) uses 'location' instead of 'loc'. plt.legend(loc='lower center', bbox_to_anchor=[0.5, -0.1]) uses a list instead of a tuple for bbox_to_anchor.

3. Final Answer:

   plt.legend(loc='lower center', bbox_to_anchor=(0.5, -0.1)) -> Option A

4. Quick Check:

   Correct loc and tuple bbox_to_anchor = plt.legend(loc='lower center', bbox_to_anchor=(0.5, -0.1)) [OK]

Hint: Use loc='position' and bbox_to_anchor=(x, y) tuple [OK]

Common Mistakes:

• Using wrong parameter name 'location' instead of 'loc'
• Swapping order in loc value like 'center lower'
• Passing list instead of tuple for bbox_to_anchor

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

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

medium

A. Legend appears inside the plot at upper left corner

B. Legend appears outside the plot at the upper left corner

C. Legend appears outside the plot at the top-right corner

D. Code raises an error due to invalid bbox_to_anchor

Solution

1. Step 1: Understand loc and bbox_to_anchor interaction

   The loc='upper left' sets the legend's anchor point to its upper left corner, and bbox_to_anchor=(1,1) places that anchor point at the top-right corner of the axes.

2. Step 2: Determine legend position

   With the legend's upper left anchored at the plot's top-right corner (1,1), the legend extends to the right, appearing outside the plot at the top-right corner.

3. Final Answer:

   Legend appears outside the plot at the top-right corner -> Option C

4. Quick Check:

   loc='upper left' + bbox_to_anchor=(1,1) = outside top-right [OK]

Hint: bbox_to_anchor moves legend outside; loc sets anchor point [OK]

Common Mistakes:

• Assuming legend stays inside plot
• Confusing loc and bbox_to_anchor roles
• Expecting an error from bbox_to_anchor values

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

import matplotlib.pyplot as plt
plt.plot([1, 2, 3], label='Data')
plt.legend(loc='bottom right', bbox_to_anchor=(1, 0))
plt.show()

medium

A. Label parameter is incorrect

B. Invalid loc value 'bottom right'

C. Missing plt.show() call

D. bbox_to_anchor must be a list, not a tuple

Solution

1. Step 1: Check valid loc values

   Matplotlib does not recognize 'bottom right' as a valid loc string. The correct value is 'lower right'.

2. Step 2: Verify other parameters

   bbox_to_anchor accepts tuples, plt.show() is present, and label syntax is correct.

3. Final Answer:

   Invalid loc value 'bottom right' -> Option B

4. Quick Check:

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

Hint: Use exact loc strings like 'lower right', not 'bottom right' [OK]

Common Mistakes:

• Using incorrect loc names
• Thinking bbox_to_anchor needs list
• Forgetting plt.show()

5. You want to place a legend outside the plot on the right side, vertically centered. Which combination of loc and bbox_to_anchor achieves this?

hard

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

B. loc='center right', bbox_to_anchor=(1, 0.5)

C. loc='center left', bbox_to_anchor=(0, 0.5)

D. loc='upper right', bbox_to_anchor=(1, 0.5)

Solution

1. Step 1: Understand legend anchor and position

   To place the legend outside on the right, the anchor point should be on the left side of the legend box, so it aligns just outside the plot.

2. Step 2: Choose correct loc and bbox_to_anchor

   loc='center left' anchors the legend's left center, and bbox_to_anchor=(1, 0.5) places it just outside the plot on the right, vertically centered.

3. Final Answer:

   loc='center left', bbox_to_anchor=(1, 0.5) -> Option A

4. Quick Check:

   Anchor left + bbox at right center = legend outside right [OK]

Hint: Anchor legend left side to place it outside right [OK]

Common Mistakes:

• Using loc='center right' which anchors wrong side
• Setting bbox_to_anchor x=0 to place outside right
• Using upper right loc for vertical center