Figure size and DPI in Matplotlib

Introduction

Figure size and DPI control how big and clear your plot looks. This helps make your charts easy to see and share.

When you want to make a plot bigger or smaller to fit a report or presentation.

When you need a high-quality image for printing or publishing.

When you want to control the resolution of your saved plot image.

When you want consistent plot sizes across multiple charts.

When you want to adjust the plot size to fit on a screen or paper.

Syntax

Matplotlib

plt.figure(figsize=(width, height), dpi=dpi_value)

figsize sets the width and height in inches.

dpi means dots per inch, controlling image resolution.

Examples

Creates a figure 6 inches wide and 4 inches tall with default DPI (usually 100).

Matplotlib

plt.figure(figsize=(6, 4))

Creates a larger figure with higher resolution for clearer details.

Matplotlib

plt.figure(figsize=(8, 6), dpi=150)

Uses default size but increases resolution for sharper image.

Matplotlib

plt.figure(dpi=200)

Sample Program

This code shows two plots with the same size but different DPI. The second plot will look sharper because it has more dots per inch.

Matplotlib

import matplotlib.pyplot as plt

# Create a figure 5x3 inches with 100 dpi
plt.figure(figsize=(5, 3), dpi=100)
plt.plot([1, 2, 3], [4, 5, 6])
plt.title('Size 5x3 inches, DPI 100')
plt.show()

# Create a figure 5x3 inches with 200 dpi
plt.figure(figsize=(5, 3), dpi=200)
plt.plot([1, 2, 3], [4, 5, 6])
plt.title('Size 5x3 inches, DPI 200')
plt.show()

Important Notes

Higher DPI means better quality but larger file size.

Figure size is in inches, so bigger numbers mean bigger plots.

If you save a figure, DPI affects the saved image resolution.

Summary

Figure size controls the physical size of the plot in inches.

DPI controls how sharp or detailed the plot image is.

Use both to make your plots look good on screen and paper.

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