Figure size and DPI in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of figure size and DPI settings

What is the size in pixels of the figure created by this code?

Matplotlib

import matplotlib.pyplot as plt
fig = plt.figure(figsize=(4, 3), dpi=100)
width, height = fig.get_size_inches() * fig.dpi
print(int(width), int(height))

A 400 300

B 4 3

C 100 75

D 40 30

💡 Hint

Remember that figure size is in inches and DPI means dots per inch.

The figure size is 4 inches by 3 inches. DPI is 100 dots per inch. So pixels = inches * DPI = 4*100=400 and 3*100=300.

❓ data_output

intermediate

DataFrame shape after resizing figure

Given this code, what is the shape of the DataFrame after plotting?

Matplotlib

import pandas as pd
import matplotlib.pyplot as plt

data = {'x': range(5), 'y': [10, 20, 15, 25, 30]}
df = pd.DataFrame(data)
fig, ax = plt.subplots(figsize=(6, 4), dpi=80)
df.plot(x='x', y='y', ax=ax)
print(df.shape)

A (5, 2)

B (6, 4)

C (80, 60)

D (4, 6)

💡 Hint

The DataFrame shape is independent of figure size.

The DataFrame has 5 rows and 2 columns, so shape is (5, 2). Figure size does not change data shape.

❓ visualization

advanced

Effect of DPI on saved image resolution

Which saved image will have the highest resolution (most pixels)?

Matplotlib

import matplotlib.pyplot as plt
fig1 = plt.figure(figsize=(5, 4), dpi=80)
fig1.savefig('image1.png')
fig2 = plt.figure(figsize=(5, 4), dpi=200)
fig2.savefig('image2.png')

A image1.png has higher resolution

B Resolution depends only on figsize, not dpi

C Both images have the same resolution

D image2.png has higher resolution

💡 Hint

DPI means dots per inch, so higher DPI means more pixels.

Both images have the same size in inches, but image2.png has dpi=200, so it has more pixels and higher resolution.

🧠 Conceptual

advanced

Understanding figure size and DPI relationship

If you double the DPI but keep the figure size the same, what happens to the image?

A The image becomes larger in inches

B The image has more pixels and looks sharper

C The image size and quality stay the same

D The image becomes smaller in pixels

💡 Hint

Think about what DPI controls in an image.

Doubling DPI increases pixels per inch, so the image has more pixels and looks sharper at the same physical size.

🔧 Debug

expert

Why does this saved figure look blurry?

This code saves a figure but the image looks blurry. What is the main cause?

Matplotlib

import matplotlib.pyplot as plt
fig = plt.figure(figsize=(3, 2), dpi=50)
plt.plot([1, 2, 3], [4, 5, 6])
fig.savefig('blurry.png')

A The plot command is missing labels

B The figsize is too large, causing blur

C The dpi is too low, causing low pixel resolution

D The savefig filename is incorrect

💡 Hint

Low DPI means fewer pixels in the image.

DPI of 50 is low, so the saved image has few pixels and looks blurry when viewed at normal size.

Practice

1. What does the figsize parameter control in a matplotlib figure?

easy

A. The physical size of the figure in inches

B. The color of the figure background

C. The resolution of the figure in dots per inch

D. The font size of the plot labels

Solution

1. Step 1: Understand the role of figsize

   The figsize parameter sets the width and height of the figure in inches.

2. Step 2: Differentiate from DPI

   DPI controls resolution, not size. Color and font size are unrelated to figsize.

3. Final Answer:

   The physical size of the figure in inches -> Option A

4. Quick Check:

   Figure size = physical dimensions [OK]

Hint: Remember: figsize = inches size, DPI = sharpness [OK]

Common Mistakes:

• Confusing DPI with figure size
• Thinking figsize controls resolution
• Mixing up color or font size with figsize

2. Which of the following is the correct way to create a matplotlib figure with size 8 inches wide and 4 inches tall?

easy

A. plt.figure(dpi=(8, 4))

B. plt.figure(size=(8, 4))

C. plt.figure(figsize=(8, 4))

D. plt.figure(shape=(8, 4))

Solution

1. Step 1: Recall correct parameter name

   The parameter to set figure size is figsize, which takes a tuple (width, height).

2. Step 2: Check syntax correctness

   Only plt.figure(figsize=(8, 4)) uses figsize correctly. Others use invalid parameter names.

3. Final Answer:

   plt.figure(figsize=(8, 4)) -> Option C

4. Quick Check:

   Use figsize=(width, height) [OK]

Hint: Use figsize=(width, height) tuple in plt.figure() [OK]

Common Mistakes:

• Using wrong parameter names like size or shape
• Passing DPI values instead of size
• Not using a tuple for figsize

3. What will be the DPI of the figure created by this code?

fig = plt.figure(figsize=(6, 3), dpi=150)

medium

A. 100

B. 150

C. 6

D. 3

Solution

1. Step 1: Identify the DPI parameter value

   The code explicitly sets dpi=150 when creating the figure.

2. Step 2: Understand DPI meaning

   DPI means dots per inch, so the figure resolution is 150 dots per inch.

3. Final Answer:

   150 -> Option B

4. Quick Check:

   DPI parameter sets resolution [OK]

Hint: DPI is set by dpi= value in plt.figure() [OK]

Common Mistakes:

• Confusing DPI with figure size
• Choosing width or height as DPI
• Assuming default DPI without checking code

4. Identify the error in this code snippet:

plt.figure(figsize=8, dpi=100)

medium

A. No error, code is correct

B. dpi must be a tuple, not an integer

C. plt.figure does not accept figsize parameter

D. figsize must be a tuple, not a single integer

Solution

1. Step 1: Check the type of figsize argument

   figsize expects a tuple (width, height), but 8 is a single integer.

2. Step 2: Verify dpi argument

   dpi accepts an integer, so dpi=100 is correct.

3. Final Answer:

   figsize must be a tuple, not a single integer -> Option D

4. Quick Check:

   figsize = (width, height) tuple [OK]

Hint: Always pass figsize as (width, height) tuple [OK]

Common Mistakes:

• Passing single int instead of tuple to figsize
• Thinking dpi needs a tuple
• Assuming figsize can be a single number

5. You want to save a plot image that is 10 inches wide and 5 inches tall with a resolution of 300 DPI. Which code snippet correctly creates and saves this figure?

hard

A. fig = plt.figure(figsize=(10, 5))
plt.plot([1, 2, 3])
fig.savefig('plot.png', dpi=300)

B. fig = plt.figure(figsize=(10, 5), dpi=100)
plt.plot([1, 2, 3])
fig.savefig('plot.png')

C. fig = plt.figure(dpi=300)
plt.plot([1, 2, 3])
fig.savefig('plot.png', figsize=(10, 5))

D. fig = plt.figure(figsize=10, dpi=300)
plt.plot([1, 2, 3])
fig.savefig('plot.png')

Solution

1. Step 1: Understand figure creation and saving

   Setting figsize in plt.figure() controls size on screen, but saving with specific DPI requires passing dpi to savefig().

2. Step 2: Analyze each option

   fig = plt.figure(figsize=(10, 5), dpi=100) plt.plot([1, 2, 3]) fig.savefig('plot.png') sets DPI in figure but saving uses default DPI, so output may differ. fig = plt.figure(figsize=(10, 5)) plt.plot([1, 2, 3]) fig.savefig('plot.png', dpi=300) sets size in figure and DPI in savefig, ensuring correct size and resolution. fig = plt.figure(dpi=300) plt.plot([1, 2, 3]) fig.savefig('plot.png', figsize=(10, 5)) wrongly passes figsize to savefig, which is invalid. fig = plt.figure(figsize=10, dpi=300) plt.plot([1, 2, 3]) fig.savefig('plot.png') passes single int to figsize, causing error.

3. Final Answer:

   fig = plt.figure(figsize=(10, 5)) plt.plot([1, 2, 3]) fig.savefig('plot.png', dpi=300) -> Option A

4. Quick Check:

   Set size in figure, DPI in savefig for correct output [OK]

Hint: Set figsize in figure, dpi in savefig to control size and resolution [OK]

Common Mistakes:

• Setting dpi only in figure but not in savefig
• Passing figsize to savefig instead of figure
• Using single int instead of tuple for figsize