Percentage labels in Matplotlib - Step-by-Step Execution

Concept Flow - Percentage labels

Start: Data and Values

↓

Create Pie Chart

↓

Calculate Percentages

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Add Percentage Labels

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Display Chart with Labels

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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.