Legend placement and styling in Matplotlib - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

A legend explains what different colors, lines, or markers in the plot represent. It helps the viewer understand the data easily.

intermediate

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

Use the loc parameter with values like 'upper left' and the bbox_to_anchor parameter to position the legend outside the plot. For example, plt.legend(loc='upper left', bbox_to_anchor=(1, 1)) places it to the right outside the plot.

beginner

What does the frameon parameter do in matplotlib legends?

The frameon parameter controls whether the legend has a border box around it. Setting frameon=False removes the box, making the legend look cleaner.

beginner

How can you change the font size of legend text in matplotlib?

Use the fontsize parameter inside plt.legend(). For example, plt.legend(fontsize='small') or plt.legend(fontsize=12) changes the text size.

intermediate

What is the effect of the ncol parameter in matplotlib legends?

The ncol parameter sets how many columns the legend entries are arranged into. For example, ncol=2 arranges legend items in two columns, which can save vertical space.

Which parameter controls the position of the legend inside or outside the plot area?

A ncol

B loc

C frameon

D fontsize

The loc parameter sets the legend's position, such as 'upper right' or 'lower left'.

How do you remove the border box around a legend in matplotlib?

A Set <code>frameon=False</code>

B Set <code>frameon=True</code>

C Set <code>fontsize=0</code>

D Set <code>loc='none'</code>

Setting frameon=False removes the border box around the legend.

What does bbox_to_anchor=(1,1) do when used with loc='upper left'?

A Places legend inside the plot at top-left

B Centers the legend in the plot

C Places legend outside the plot to the right at top

D Removes the legend

It places the legend outside the plot area to the right, aligned at the top.

Which parameter changes the number of columns in a legend?

A ncol

B fontsize

C loc

D frameon

The ncol parameter sets how many columns the legend uses.

How can you make legend text smaller in matplotlib?

A Set <code>ncol=2</code>

B Set <code>frameon=False</code>

C Set <code>loc='lower right'</code>

D Set <code>fontsize='small'</code>

Using fontsize='small' or a smaller numeric value reduces the legend text size.

Explain how to place a legend outside the plot area and style it with no border and smaller font.

Think about loc, bbox_to_anchor, frameon, and fontsize parameters.

Describe how to arrange legend items in two columns and why you might want to do that.

Focus on legend layout and space management.

Practice

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

easy

A. The type of plot line used

B. The color of the legend text

C. The size of the plot markers

D. The position of the legend on the plot

Solution

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

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

2. Step 2: Differentiate from other styling options

   Other options control color, size, or line style, but loc only controls position.

3. Final Answer:

   The position of the legend on the plot -> Option D

4. Quick Check:

   Legend position = loc [OK]

Hint: Remember: loc means location of legend [OK]

Common Mistakes:

• Confusing loc with color or font size
• Thinking loc changes plot markers
• Assuming loc controls line style

2. Which of the following is the correct syntax to place a legend at the lower left corner in matplotlib?

easy

A. plt.legend(loc='lower left')

B. plt.legend(location='lower left')

C. plt.legend(loc='lower_left')

D. plt.legend(position='lower left')

Solution

1. Step 1: Identify the correct parameter name

   The correct parameter to set legend position is loc, not location or position.

2. Step 2: Use the correct string value

   The valid string is 'lower left' with a space, not 'lower_left' with underscore.

3. Final Answer:

   plt.legend(loc='lower left') -> Option A

4. Quick Check:

   Correct syntax uses loc='lower left' [OK]

Hint: Use loc='lower left' exactly, no underscores [OK]

Common Mistakes:

• Using 'location' instead of 'loc'
• Using underscores instead of spaces
• Using 'position' parameter which doesn't exist

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

import matplotlib.pyplot as plt
plt.plot([1, 2, 3], label='Line A')
plt.plot([3, 2, 1], label='Line B')
plt.legend(loc='upper center', shadow=True, fontsize='small')
plt.show()

medium

A. Code will raise an error due to invalid fontsize

B. Legend appears at the top center with a shadow and small font

C. Legend appears inside the plot area without shadow

D. Legend appears at the bottom right with default font size

Solution

1. Step 1: Analyze legend parameters

   The loc='upper center' places the legend at the top center. shadow=True adds a shadow behind the legend box. fontsize='small' sets the font size smaller than default.

2. Step 2: Confirm no errors in parameters

   All parameters are valid and supported by matplotlib.

3. Final Answer:

   Legend appears at the top center with a shadow and small font -> Option B

4. Quick Check:

   loc + shadow + fontsize = top center, shadow, small font [OK]

Hint: Check loc for position, shadow=True adds shadow, fontsize sets text size [OK]

Common Mistakes:

• Assuming fontsize='small' causes error
• Confusing loc='upper center' with bottom right
• Ignoring shadow parameter effect

4. Identify the error in this code snippet for legend placement:

import matplotlib.pyplot as plt
plt.plot([1, 2, 3], label='A')
plt.plot([3, 2, 1], label='B')
plt.legend(loc='bottomright')
plt.show()

medium

A. plt.legend() must be called before plt.plot()

B. Missing label parameter in plot calls

C. The loc value 'bottomright' is invalid; should be 'lower right'

D. plt.show() is missing parentheses

Solution

1. Step 1: Check the loc parameter value

   The valid location string is 'lower right', not 'bottomright'. 'bottomright' is not recognized by matplotlib.

2. Step 2: Verify other parts of the code

   Labels are correctly set, plt.legend() can be called after plotting, and plt.show() has parentheses.

3. Final Answer:

   The loc value 'bottomright' is invalid; should be 'lower right' -> Option C

4. Quick Check:

   Valid loc strings = 'lower right' not 'bottomright' [OK]

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

Common Mistakes:

• Using invalid loc strings
• Thinking plt.legend() must come before plt.plot()
• Forgetting parentheses in plt.show()

5. You want to place a legend outside the plot on the right side with a title 'Legend', font size 12, and a shadow. Which code snippet achieves this correctly?

hard

A. plt.legend(loc='center right', bbox_to_anchor=(1, 0.5), title='Legend', fontsize='12pt', shadow=True)

B. plt.legend(loc='right side', bbox_to_anchor=(1, 0.5), title='Legend', fontsize=12, shadow=True)

C. plt.legend(loc='center left', bbox_to_anchor=(1, 0.5), title='Legend', fontsize=12, shadow=True)

D. plt.legend(loc='outside right', bbox_to_anchor=(1, 0.5), title='Legend', fontsize=12, shadow=True)

Solution

1. Step 1: Understand placing legend outside plot

   Using loc='center right' with bbox_to_anchor=(1, 0.5) places the legend outside on the right side centered vertically.

2. Step 2: Check styling parameters

   Title 'Legend', font size 12 (string with units '12pt'), and shadow=True are correctly set. loc='right side' and loc='outside right' are invalid.

3. Final Answer:

   plt.legend(loc='center right', bbox_to_anchor=(1, 0.5), title='Legend', fontsize='12pt', shadow=True) -> Option A

4. Quick Check:

   loc + bbox_to_anchor + title + fontsize + shadow = correct legend outside right [OK]

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

Common Mistakes:

• Using invalid loc like 'right side' or 'outside right'
• Passing fontsize as integer without units
• Forgetting bbox_to_anchor for outside placement