Percentage labels in Matplotlib - Mini Project: Build & Apply

Percentage labels

📖 Scenario: You work in a small bakery and want to show how much each type of pastry contributes to total sales. You want to create a pie chart that shows the percentage share of each pastry type.

🎯 Goal: Create a pie chart using matplotlib that displays the percentage labels for each pastry type's sales.

📋 What You'll Learn

Create a dictionary called pastry_sales with exact entries for pastry types and their sales numbers.

Create a variable called labels that holds the pastry names from the dictionary.

Create a variable called sizes that holds the sales numbers from the dictionary.

Use matplotlib.pyplot.pie to create a pie chart with percentage labels shown.

Print the pie chart to display it.

💡 Why This Matters

🌍 Real World

Pie charts with percentage labels are useful to show parts of a whole in sales reports, surveys, and business presentations.

💼 Career

Data analysts and business intelligence professionals often create pie charts to communicate data insights clearly to stakeholders.

1

Create the pastry sales data

Create a dictionary called pastry_sales with these exact entries: 'Croissant': 120, 'Muffin': 90, 'Danish': 60, 'Eclair': 30.

Matplotlib

# Create the pastry_sales dictionary with exact values
# Your code here

Hint

Use curly braces {} to create the dictionary with the exact pastry names and sales numbers.

2

Extract labels and sizes

Create a list called labels that contains the keys from pastry_sales. Create a list called sizes that contains the values from pastry_sales.

Matplotlib

pastry_sales = {'Croissant': 120, 'Muffin': 90, 'Danish': 60, 'Eclair': 30}
# Create labels list from pastry_sales keys
# Create sizes list from pastry_sales values
# Your code here

Hint

Use list() with pastry_sales.keys() and pastry_sales.values() to get the lists.

3

Create the pie chart with percentage labels

Import matplotlib.pyplot as plt. Use plt.pie with sizes and labels. Add the argument autopct='%1.1f%%' to show percentage labels on the pie slices.

Matplotlib

pastry_sales = {'Croissant': 120, 'Muffin': 90, 'Danish': 60, 'Eclair': 30}
labels = list(pastry_sales.keys())
sizes = list(pastry_sales.values())
# Import matplotlib.pyplot as plt
# Create pie chart with sizes, labels, and autopct='%1.1f%%'
# Your code here

Hint

Use import matplotlib.pyplot as plt and then plt.pie() with the correct arguments.

4

Display the pie chart

Use plt.show() to display the pie chart.

Matplotlib

import matplotlib.pyplot as plt

pastry_sales = {'Croissant': 120, 'Muffin': 90, 'Danish': 60, 'Eclair': 30}
labels = list(pastry_sales.keys())
sizes = list(pastry_sales.values())

plt.pie(sizes, labels=labels, autopct='%1.1f%%')
# Display the pie chart
# Your code here

Hint

Call plt.show() to open the window with the pie chart.

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