Percentage labels in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of pie chart with percentage labels

What will be the output of this code snippet that creates a pie chart with percentage labels?

Matplotlib

import matplotlib.pyplot as plt
sizes = [15, 30, 45, 10]
labels = ['A', 'B', 'C', 'D']
plt.pie(sizes, labels=labels, autopct='%1.1f%%')
plt.show()

A A pie chart with four slices labeled A, B, C, D and percentages 15%, 30%, 45%, 10% without decimal places

B A pie chart with four slices labeled A, B, C, D but no percentage labels shown

C A pie chart with four slices labeled A, B, C, D and percentages 15.0%, 30.0%, 45.0%, 10.0% displayed on each slice

D A pie chart with four slices but labels are numbers 0, 1, 2, 3 instead of A, B, C, D

💡 Hint

Look at the autopct parameter format string '%1.1f%%' to understand the percentage format.

The autopct='%1.1f%%' formats the percentage with one decimal place and adds a percent sign. Labels are shown as given in the labels list.

🧠 Conceptual

intermediate

Understanding autopct parameter in pie charts

What does the autopct parameter do in matplotlib's pie chart function?

A It formats and displays the percentage value on each pie slice

B It sets the color of each pie slice

C It controls the size of the pie chart

D It adds a legend to the pie chart

💡 Hint

Think about what 'pct' in autopct might stand for.

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

🔧 Debug

advanced

Identify the error in percentage label formatting

What error will this code produce when trying to add percentage labels to a pie chart? import matplotlib.pyplot as plt sizes = [10, 20, 30] labels = ['X', 'Y', 'Z'] plt.pie(sizes, labels=labels, autopct='%1.f%%') plt.show()

A ValueError because '%1.f%%' is an invalid format specifier

B No error; the pie chart shows percentages without decimal places

C TypeError because autopct expects a function, not a string

D SyntaxError due to incorrect format string in autopct

💡 Hint

Check if '%1.f%%' is a valid Python format specifier for floats.

The format specifier '%1.f%%' is invalid because after the decimal point '.', a precision digit is required (e.g., '%1.0f%%'). This causes a ValueError.

❓ data_output

advanced

Resulting labels with custom autopct function

Given this code, what is the output of the labels shown on the pie chart slices? import matplotlib.pyplot as plt sizes = [25, 25, 50] labels = ['Red', 'Blue', 'Green'] def func(pct): return f'{pct:.0f}%% of total' plt.pie(sizes, labels=labels, autopct=func) plt.show()

A Slices labeled 'Red', 'Blue', 'Green' with percentages '25.0%', '25.0%', '50.0%'

B Slices labeled 'Red', 'Blue', 'Green' with percentages '25%% of total', '25%% of total', '50%% of total'

C Slices labeled 'Red', 'Blue', 'Green' with no percentage labels

D Slices labeled 'Red', 'Blue', 'Green' with percentages '25% of total', '25% of total', '50% of total'

💡 Hint

Look at the custom function formatting the percentage string.

The custom function formats the percentage with no decimals and adds ' of total' text after the percent sign.

🚀 Application

expert

Creating a pie chart with percentage labels and exploded slice

Which code snippet correctly creates a pie chart with percentage labels showing one decimal place, labels for each slice, and the largest slice exploded outward?

A

import matplotlib.pyplot as plt
sizes = [10, 20, 30, 40]
labels = ['A', 'B', 'C', 'D']
explode = [0.1, 0, 0, 0]
plt.pie(sizes, labels=labels, autopct='%1.1f%%', explode=explode)
plt.show()

B

import matplotlib.pyplot as plt
sizes = [10, 20, 30, 40]
labels = ['A', 'B', 'C', 'D']
explode = [0, 0, 0, 0.1]
plt.pie(sizes, labels=labels, autopct='%1.1f%%', explode=explode)
plt.show()

C

import matplotlib.pyplot as plt
sizes = [10, 20, 30, 40]
labels = ['A', 'B', 'C', 'D']
explode = [0, 0, 0, 0.1]
plt.pie(sizes, labels=labels, autopct='%1.0f%%', explode=explode)
plt.show()

D

import matplotlib.pyplot as plt
sizes = [10, 20, 30, 40]
labels = ['A', 'B', 'C', 'D']
explode = [0, 0, 0, 0.1]
plt.pie(sizes, labels=labels, autopct='%1.1f', explode=explode)
plt.show()

💡 Hint

The largest slice is 40, so explode should highlight that slice. Check the autopct format for one decimal place and percent sign.

Option B explodes the largest slice (index 3) with 0.1 offset, uses autopct='%1.1f%%' to show one decimal place and percent sign correctly.

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