Why pie charts show proportions in Matplotlib - Quick Recap

Recall & Review

beginner

What does each slice in a pie chart represent?

Each slice in a pie chart represents a part of the whole, showing the proportion or percentage of that category compared to the total.

beginner

Why are pie charts useful for showing proportions?

Pie charts visually display how different parts make up a whole, making it easy to compare relative sizes at a glance.

intermediate

How does matplotlib create a pie chart to show proportions?

Matplotlib divides a circle into slices where each slice's angle corresponds to the proportion of that category's value to the total sum of all values.

beginner

What is the importance of the total sum in pie charts?

The total sum is important because each slice's size depends on the fraction of its value over the total sum, ensuring the chart accurately shows proportions.

beginner

Can pie charts show exact values or only proportions?

Pie charts primarily show proportions, but labels or legends can display exact values alongside the visual slices.

What does the size of a slice in a pie chart represent?

A The proportion of that category to the whole

B The total number of categories

C The color of the category

D The order of data input

Each slice size shows how big that category is compared to the total, representing its proportion.

Why is the total sum of values important in a pie chart?

A It helps calculate each slice's proportion

B It determines the size of the whole circle

C It is not important

D It changes the colors of slices

The total sum is used to find what fraction each category is of the whole, which sets slice sizes.

Which matplotlib function is used to create pie charts?

A plt.bar()

B plt.pie()

C plt.scatter()

D plt.plot()

The plt.pie() function draws pie charts in matplotlib.

What is a limitation of pie charts?

A They can only show proportions, not exact values

B They cannot show proportions

C They do not use colors

D They are hard to read with many categories

Pie charts become hard to interpret when there are too many slices, making comparison difficult.

How can exact values be shown on a pie chart?

A By increasing the circle size

B By changing slice colors

C By adding labels or legends

D By sorting the data

Labels or legends can display exact numbers alongside the proportional slices.

Explain why pie charts are a good choice for showing proportions in data.

Think about how a pizza is divided into slices.

Describe how matplotlib calculates the size of each slice in a pie chart.

Consider how a circle is split based on data proportions.

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