Why pie charts show proportions in Matplotlib - See It in Action

Why Pie Charts Show Proportions

📖 Scenario: Imagine you run a small bakery and want to understand which types of pastries sell the most. You have sales data for different pastry types and want to visualize their share of total sales.

🎯 Goal: You will create a pie chart using matplotlib to show the proportions of pastry sales. This will help you see which pastries are most popular at a glance.

📋 What You'll Learn

Create a dictionary called pastry_sales with exact pastry names and sales numbers

Create a variable called total_sales that sums all sales

Use a for loop with variables pastry and sales to calculate proportions

Use matplotlib.pyplot.pie() to create a pie chart showing proportions

Print the proportions dictionary to see the share of each pastry

💡 Why This Matters

🌍 Real World

Pie charts are used in business to quickly show how parts make up a whole, like sales shares or budget spending.

💼 Career

Data analysts and scientists use pie charts to communicate proportions clearly to stakeholders and help make decisions.

1

Create the pastry sales data

Create a dictionary called pastry_sales with these exact entries: 'Croissant': 120, 'Muffin': 80, 'Danish': 50, 'Eclair': 30

Matplotlib

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

Hint

Use curly braces {} to create a dictionary with pastry names as keys and sales numbers as values.

2

Calculate total sales

Create a variable called total_sales that sums all the sales values in pastry_sales using the sum() function and .values() method

Matplotlib

pastry_sales = {'Croissant': 120, 'Muffin': 80, 'Danish': 50, 'Eclair': 30}
# Create total_sales by summing pastry_sales values
# Your code here

Hint

Use sum(pastry_sales.values()) to add all sales numbers.

3

Calculate proportions of each pastry

Create an empty dictionary called proportions. Use a for loop with variables pastry and sales to iterate over pastry_sales.items(). Inside the loop, calculate the proportion as sales / total_sales and store it in proportions[pastry]

Matplotlib

pastry_sales = {'Croissant': 120, 'Muffin': 80, 'Danish': 50, 'Eclair': 30}
total_sales = sum(pastry_sales.values())
# Create proportions dictionary and fill it with sales proportions
# Your code here

Hint

Use a for loop to go through each pastry and calculate its share by dividing sales by total_sales.

4

Create and show the pie chart

Import matplotlib.pyplot as plt. Use plt.pie() with the sales values from pastry_sales.values() and labels from pastry_sales.keys(). Add autopct='%1.1f%%' to show percentages. Use plt.title() to add the title 'Pastry Sales Proportions'. Finally, call plt.show() to display the chart. Also, print the proportions dictionary.

Matplotlib

pastry_sales = {'Croissant': 120, 'Muffin': 80, 'Danish': 50, 'Eclair': 30}
total_sales = sum(pastry_sales.values())
proportions = {}
for pastry, sales in pastry_sales.items():
    proportions[pastry] = sales / total_sales
# Import matplotlib.pyplot as plt and create the pie chart
# Print the proportions dictionary
# Your code here

Hint

Remember to import matplotlib.pyplot as plt. Use plt.pie() with sales values and labels, add autopct for percentages, then show the plot with plt.show(). Finally, print the proportions dictionary.

Practice

1. Why do pie charts show slices of different sizes?

easy

A. Because each slice represents a proportion of the total

B. Because slices are randomly sized

C. Because all slices must be equal

D. Because slices represent unrelated data points

Solution

1. Step 1: Understand what pie chart slices represent

   Each slice shows a part of the whole data set.

2. Step 2: Connect slice size to proportion

   The size of each slice matches how big that part is compared to the total.

3. Final Answer:

   Because each slice represents a proportion of the total -> Option A

4. Quick Check:

   Slice size = proportion [OK]

Hint: Remember: pie slices show parts of a whole [OK]

Common Mistakes:

• Thinking slices are random sizes
• Assuming all slices are equal
• Believing slices show unrelated data

2. Which matplotlib function is used to create a pie chart?

easy

A. plt.bar()

B. plt.pie()

C. plt.scatter()

D. plt.plot()

Solution

1. Step 1: Recall matplotlib plotting functions

   Matplotlib has specific functions for different chart types.

2. Step 2: Identify the pie chart function

   The function plt.pie() is designed to create pie charts.

3. Final Answer:

   plt.pie() -> Option B

4. Quick Check:

   Pie chart = plt.pie() [OK]

Hint: Pie chart uses plt.pie() function [OK]

Common Mistakes:

• Using plt.bar() for pie charts
• Confusing plt.scatter() with pie charts
• Trying plt.plot() for pie charts

3. What will the following code output?

import matplotlib.pyplot as plt
sizes = [30, 70]
plt.pie(sizes, labels=['A', 'B'])
plt.show()

medium

A. A line plot connecting points 30 and 70

B. A bar chart with two bars of heights 30 and 70

C. An error because labels are missing

D. A pie chart with two slices sized 30% and 70%

Solution

1. Step 1: Analyze the plt.pie() call

   The sizes list has two values: 30 and 70, which sum to 100.

2. Step 2: Understand labels and output

   Labels 'A' and 'B' correspond to slices sized 30% and 70% of the pie.

3. Final Answer:

   A pie chart with two slices sized 30% and 70% -> Option D

4. Quick Check:

   Pie slices match sizes list [OK]

Hint: Pie slices match sizes list values [OK]

Common Mistakes:

• Confusing pie chart with bar chart
• Expecting error due to labels
• Thinking it creates a line plot

4. Identify the error in this pie chart code:

import matplotlib.pyplot as plt
sizes = [20, 30, 50]
plt.pie(sizes, labels=['X', 'Y'])
plt.show()

medium

A. The sizes list has invalid values

B. plt.pie() cannot plot more than two slices

C. The labels list length does not match sizes list length

D. plt.show() is missing

Solution

1. Step 1: Check sizes and labels lengths

   Sizes has 3 values, labels has only 2 labels.

2. Step 2: Understand matplotlib requirements

   Labels must match the number of slices to label each slice properly.

3. Final Answer:

   The labels list length does not match sizes list length -> Option C

4. Quick Check:

   Labels count = sizes count [OK]

Hint: Labels count must equal sizes count [OK]

Common Mistakes:

• Thinking sizes values are invalid
• Believing plt.pie() limits slices
• Forgetting plt.show() is needed

5. You want to show proportions of sales for 4 products using a pie chart. The sales are [100, 200, 300, 400]. How do you ensure the pie chart correctly shows proportions?

hard

A. Pass the sales list directly to plt.pie() and add labels for each product

B. Normalize sales to percentages before passing to plt.pie()

C. Use plt.bar() instead because pie charts need percentages

D. Divide each sales value by 1000 before plotting

Solution

1. Step 1: Understand how plt.pie() works with data

   Matplotlib automatically calculates proportions from raw values.

2. Step 2: Confirm labels and data match

   Passing raw sales with matching labels shows correct slice sizes.

3. Final Answer:

   Pass the sales list directly to plt.pie() and add labels for each product -> Option A

4. Quick Check:

   Raw values work; no manual normalization needed [OK]

Hint: Pass raw values; matplotlib calculates proportions [OK]

Common Mistakes:

• Thinking manual percentage calculation is needed
• Using bar chart instead of pie chart
• Scaling values unnecessarily