Figure size and DPI in Matplotlib - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'figure size' control in a matplotlib plot?

Figure size controls the width and height of the plot in inches. It determines how big the plot will appear on the screen or in a saved image.

beginner

What is DPI in matplotlib and why is it important?

DPI stands for Dots Per Inch. It controls the resolution of the figure, meaning how many dots (pixels) are used per inch. Higher DPI means sharper images.

beginner

How do you set the figure size to 8 inches wide and 6 inches tall in matplotlib?

Use plt.figure(figsize=(8, 6)) to set the figure size to 8 inches wide and 6 inches tall.

intermediate

How does increasing DPI affect the saved image size and quality?

Increasing DPI increases the number of pixels per inch, making the image sharper and larger in pixel dimensions. The file size usually becomes bigger too.

intermediate

What happens if you set a large figure size but low DPI?

The plot will be physically large but may look blurry or pixelated because the resolution (DPI) is low.

In matplotlib, what does figsize=(10, 5) mean?

A The figure has 10 dots per inch horizontally and 5 vertically

B The figure is 10 inches wide and 5 inches tall

C The figure is 10 pixels wide and 5 pixels tall

D The figure will be saved with 10 DPI

figsize sets the width and height in inches, so (10, 5) means 10 inches wide and 5 inches tall.

What does a higher DPI value do to a matplotlib figure?

A Increases the resolution and sharpness of the figure

B Makes the figure physically bigger on screen

C Decreases the number of pixels in the figure

D Changes the color of the figure

Higher DPI means more dots per inch, so the figure is sharper and has higher resolution.

Which code sets a figure size of 6x4 inches with 150 DPI?

A plt.figure(figsize=(6,4), dpi=150)

B plt.figure(size=(6,4), dpi=150)

C plt.figure(figsize=(150,6), dpi=4)

D plt.figure(dpi=(6,4), figsize=150)

The correct syntax is figsize=(width, height) in inches and dpi=resolution.

If you want a small image file but still clear, what should you do?

A Use large figure size and low DPI

B Use small figure size and low DPI

C Use small figure size and high DPI

D Use large figure size and high DPI

Small figure size reduces pixel count, and high DPI keeps the image sharp.

What unit is used for figure size in matplotlib?

A Points

B Centimeters

C Pixels

D Inches

Matplotlib uses inches for figure size.

Explain how figure size and DPI work together to affect the appearance and quality of a matplotlib plot.

Think about how size and resolution affect a photo's clarity.

Describe how you would create a matplotlib figure that is 10 inches wide, 5 inches tall, and has a resolution of 200 DPI.

Remember figsize is a tuple of inches, dpi is dots per inch.

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