Percentage labels in Matplotlib - Step-by-Step Execution

Concept Flow - Percentage labels

Start: Data and Values

↓

Create Pie Chart

↓

Calculate Percentages

↓

Add Percentage Labels

↓

Display Chart with Labels

↓

End

The flow shows how data values are used to create a pie chart, calculate percentages, add labels, and display the final chart.

Execution Sample

Matplotlib

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

This code creates a pie chart with four slices and adds percentage labels on each slice.

Execution Table

Step	Action	Input/State	Output/Result
1	Define sizes list	sizes = [15, 30, 45, 10]	sizes stored
2	Call plt.pie()	sizes and labels=['A','B','C','D'], autopct='%1.1f%%'	Pie chart slices created with labels
3	Calculate total	Sum of sizes = 100	Total = 100
4	Calculate percentage for each slice	Each size / total * 100	Percentages: 15%, 30%, 45%, 10%
5	Format percentage labels	Using '%1.1f%%'	Labels: '15.0%', '30.0%', '45.0%', '10.0%'
6	Add labels to pie slices	Labels and percentages	Pie chart slices labeled with percentages
7	Display chart	Pie chart with labels	Pie chart shown on screen
8	End	Chart displayed	Execution complete

💡 All slices labeled and chart displayed, execution ends.

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	Final
sizes	undefined	[15, 30, 45, 10]	[15, 30, 45, 10]	[15, 30, 45, 10]	[15, 30, 45, 10]
total	undefined	undefined	100	100	100
percentages	undefined	undefined	undefined	[15.0, 30.0, 45.0, 10.0]	[15.0, 30.0, 45.0, 10.0]
labels	undefined	undefined	undefined	undefined	['15.0%', '30.0%', '45.0%', '10.0%']

Key Moments - 3 Insights

Why do the percentage labels add up to 100% even if the sizes are arbitrary numbers?

Because the code calculates the total sum of all sizes (step 3) and then computes each slice's percentage relative to that total (step 4), ensuring the labels represent parts of the whole.

What does the format string '%1.1f%%' mean in autopct?

It formats the percentage to show one digit before and one digit after the decimal point, followed by a percent sign, as seen in step 5 of the execution table.

Can the autopct parameter be a function instead of a format string?

Yes, autopct can be a function that receives the percentage and returns a custom label string, but here we use a format string for simplicity (step 5).

Visual Quiz - 3 Questions

Look at the execution table, what is the total sum of sizes calculated at step 3?

A 90

B 120

C 100

D None

💡 Hint

Check the 'Calculate total' row in the execution_table at step 3.

At which step are the percentage labels formatted as strings like '15.0%'?

A Step 5

B Step 4

C Step 2

D Step 6

💡 Hint

Look for the step where formatting with '%1.1f%%' is applied in the execution_table.

If the sizes list changed to [10, 20, 30, 40], what would happen to the percentages in the variable_tracker?

A They would remain the same as [15.0, 30.0, 45.0, 10.0]

B They would change to [10.0, 20.0, 30.0, 40.0] and sum to 100

C They would change to [10.0, 20.0, 30.0, 40.0]

D They would change to [10.0, 20.0, 30.0, 40.0] but not sum to 100

💡 Hint

Percentages are calculated as each size divided by total sum times 100, so they always sum to 100 (see variable_tracker).

Concept Snapshot

Percentage labels in matplotlib pie charts:
- Use plt.pie() with autopct parameter
- autopct='%1.1f%%' formats labels as percentages
- Percentages are calculated from slice sizes
- Labels show on each pie slice automatically
- Useful for visualizing parts of a whole

Full Transcript

This visual execution traces how matplotlib creates a pie chart with percentage labels. First, a list of sizes is defined. Then plt.pie() is called with labels and autopct format string. The total of sizes is calculated to find the whole. Each slice's percentage is computed by dividing its size by the total and multiplying by 100. These percentages are formatted as strings like '15.0%'. The labels are added to the pie slices. Finally, the chart is displayed. Key points include how percentages always sum to 100 and how the autopct string controls label formatting.

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