Percentage labels in Matplotlib

Introduction

Percentage labels help show parts of a whole clearly in charts. They make it easy to see how big each piece is compared to the total.

Showing how much each category contributes to total sales in a pie chart.

Displaying the percentage of votes each candidate got in an election.

Visualizing the share of different expenses in a monthly budget.

Comparing proportions of different product types sold in a store.

Syntax

Matplotlib

plt.pie(data, labels=labels, autopct='%1.1f%%')

autopct adds percentage labels on the pie slices.

The format string '%1.1f%%' shows one decimal place and a percent sign.

Examples

Shows percentages with no decimal places.

Matplotlib

plt.pie([30, 70], labels=['A', 'B'], autopct='%1.0f%%')

Shows percentages with two decimal places.

Matplotlib

plt.pie([10, 20, 30], labels=['X', 'Y', 'Z'], autopct='%1.2f%%')

Uses a function to format percentage labels.

Matplotlib

plt.pie([5, 15], labels=['Cat1', 'Cat2'], autopct=lambda p: f'{p:.1f}%')

Sample Program

This code creates a pie chart showing the percentage of each fruit type. The autopct adds percentage labels with one decimal place.

Matplotlib

import matplotlib.pyplot as plt

sizes = [40, 35, 25]
labels = ['Apples', 'Bananas', 'Cherries']

plt.pie(sizes, labels=labels, autopct='%1.1f%%')
plt.title('Fruit Distribution')
plt.show()

Important Notes

Percentage labels help viewers understand data proportions quickly.

You can customize the format string to show more or fewer decimals.

Using a function for autopct allows full control over label text.

Summary

Percentage labels show how big each slice is compared to the whole.

Use autopct in plt.pie() to add them easily.

Format strings or functions control how percentages appear.

Practice

1. What does the autopct parameter do in plt.pie() when creating a pie chart?

easy

A. It adds percentage labels to each slice of the pie chart.

B. It changes the colors of the pie slices.

C. It sets the title of the pie chart.

D. It adjusts the size of the pie chart.

Solution

1. Step 1: Understand the role of autopct in plt.pie()

   The autopct parameter is used to display the percentage value on each slice of the pie chart.

2. Step 2: Compare with other parameters

   Other parameters like colors or title do not control percentage labels; only autopct does.

3. Final Answer:

   It adds percentage labels to each slice of the pie chart. -> Option A

4. Quick Check:

   autopct = percentage labels [OK]

Hint: Remember autopct shows percentages on pie slices [OK]

Common Mistakes:

• Confusing autopct with color or title parameters
• Thinking autopct changes chart size
• Ignoring autopct and missing percentage labels

2. Which of the following is the correct syntax to show percentages with one decimal place in a pie chart using plt.pie()?

easy

A. plt.pie(data, autopct='%.1f%')

B. plt.pie(data, autopct='%f1.1%')

C. plt.pie(data, autopct='%1.1f%%')

D. plt.pie(data, autopct='%1.f%%')

Solution

1. Step 1: Recall the format string for percentages

   The correct format for one decimal place is '%1.1f%%' where '%1.1f' formats the float and '%%' prints the percent sign.

2. Step 2: Check each option's format

   plt.pie(data, autopct='%1.1f%%') matches the correct format. Options A, B, and D have misplaced or missing percent signs or incorrect format specifiers.

3. Final Answer:

   plt.pie(data, autopct='%1.1f%%') -> Option C

4. Quick Check:

   Format string '%1.1f%%' = correct percentage format [OK]

Hint: Use '%1.1f%%' to show one decimal percent [OK]

Common Mistakes:

• Forgetting to double the percent sign '%%'
• Misplacing the decimal point in format
• Using incorrect format specifiers like '%f1.1%'

3. What will be the output percentages shown on the pie chart slices for the following code?

import matplotlib.pyplot as plt
sizes = [10, 20, 30]
plt.pie(sizes, autopct='%1.0f%%')
plt.show()

medium

A. 10%, 20%, 30%

B. 17%, 33%, 50%

C. 25%, 50%, 75%

D. 33%, 33%, 33%

Solution

1. Step 1: Calculate total and percentages

   Total = 10 + 20 + 30 = 60. Percentages: 10/60=16.67%, 20/60=33.33%, 30/60=50%.

2. Step 2: Apply autopct format '%1.0f%%'

   This rounds percentages to nearest whole number: 17%, 33%, 50%.

3. Final Answer:

   17%, 33%, 50% -> Option B

4. Quick Check:

   Rounded percentages = 17%, 33%, 50% [OK]

Hint: Sum slices, divide each, round with autopct [OK]

Common Mistakes:

• Using raw values instead of percentages
• Not rounding percentages correctly
• Assuming equal slices without calculation

4. Identify the error in this code snippet that tries to add percentage labels to a pie chart:

import matplotlib.pyplot as plt
sizes = [40, 60]
plt.pie(sizes, autopct='%1.1f%')
plt.show()

medium

A. The format string '%1.1f%' is missing an extra percent sign.

B. The sizes list should be a numpy array.

C. plt.show() is missing parentheses.

D. autopct should be set to True instead of a format string.

Solution

1. Step 1: Check the autopct format string

   The format string '%1.1f%' is incorrect because a single '%' is interpreted as a format specifier start, so to print a percent sign, '%%' is needed.

2. Step 2: Verify other parts of the code

   The sizes list can be a list, plt.show() has parentheses, and autopct expects a format string, not True.

3. Final Answer:

   The format string '%1.1f%' is missing an extra percent sign. -> Option A

4. Quick Check:

   Use '%%' to print percent sign in autopct [OK]

Hint: Double percent signs '%%' to show % in autopct [OK]

Common Mistakes:

• Using single '%' instead of '%%' in format string
• Thinking autopct accepts boolean True
• Assuming list must be numpy array

5. You want to create a pie chart with slices labeled by percentage, but only show percentages for slices larger than 20%. Which autopct function below correctly implements this?

hard

A. lambda p: f'{p:.1f}%'

B. lambda p: f'{p:.1f}%' if p < 20 else ''

C. lambda p: f'{p:.0f}%' if p >= 20 else '0%'

D. lambda p: f'{p:.1f}%' if p > 20 else ''

Solution

1. Step 1: Understand the requirement

   Show percentage labels only if slice percentage is greater than 20%, else show nothing.

2. Step 2: Analyze each lambda function

   lambda p: f'{p:.1f}%' if p > 20 else '' returns formatted percentage if p > 20, else empty string. lambda p: f'{p:.1f}%' if p < 20 else '' does the opposite. lambda p: f'{p:.0f}%' if p >= 20 else '0%' shows '0%' for smaller slices, which is not desired. lambda p: f'{p:.1f}%' shows all percentages.

3. Final Answer:

   lambda p: f'{p:.1f}%' if p > 20 else '' -> Option D

4. Quick Check:

   Show label only if p > 20% = lambda p: f'{p:.1f}%' if p > 20 else '' [OK]

Hint: Use lambda with condition to filter percentages [OK]

Common Mistakes:

• Showing percentages for all slices
• Using wrong comparison operator (< instead of >)
• Returning '0%' instead of empty string for small slices