Saving figures to files in Matplotlib - Mini Project: Build & Apply

Saving figures to files

📖 Scenario: You are working on a small data science project where you create simple charts to understand sales data. You want to save these charts as image files so you can share them with your team.

🎯 Goal: Create a bar chart from sales data and save the figure as a PNG file on your computer.

📋 What You'll Learn

Create a dictionary with sales data for three products

Set a filename variable for the image file

Use matplotlib to create a bar chart from the sales data

Save the figure to the file using the filename variable

Print a confirmation message with the filename

💡 Why This Matters

🌍 Real World

Saving charts as image files is common when sharing data insights with others or including visuals in reports.

💼 Career

Data scientists and analysts often save figures to files to document their findings and communicate results clearly.

1

Create sales data dictionary

Create a dictionary called sales with these exact entries: 'Apples': 30, 'Bananas': 45, 'Cherries': 25.

Matplotlib

# Create the sales dictionary with exact entries
# Your code here

Hint

Use curly braces to create a dictionary with keys and values separated by colons.

2

Set filename for saving figure

Create a variable called filename and set it to the string 'sales_chart.png'.

Matplotlib

sales = {'Apples': 30, 'Bananas': 45, 'Cherries': 25}
# Create a filename variable with the value 'sales_chart.png'
# Your code here

Hint

Use a string variable to store the filename for the image.

3

Create bar chart with matplotlib

Import matplotlib.pyplot as plt. Use plt.bar() with sales.keys() and sales.values() to create a bar chart. Then use plt.savefig(filename) to save the figure to the file.

Matplotlib

sales = {'Apples': 30, 'Bananas': 45, 'Cherries': 25}
filename = 'sales_chart.png'
# Import matplotlib.pyplot as plt
# Create a bar chart using sales keys and values
# Save the figure to the filename
# Your code here

Hint

Use plt.bar() to create the bar chart and plt.savefig() to save it.

4

Print confirmation message

Write a print statement that outputs: "Figure saved as sales_chart.png" using the filename variable inside an f-string.

Matplotlib

import matplotlib.pyplot as plt

sales = {'Apples': 30, 'Bananas': 45, 'Cherries': 25}
filename = 'sales_chart.png'

plt.bar(sales.keys(), sales.values())
plt.savefig(filename)
# Print confirmation message with filename
# Your code here

Hint

Use print(f"Figure saved as {filename}") to show the message.

Practice

1. What does the plt.savefig() function do in matplotlib?

easy

A. It saves the current figure to a file on your computer.

B. It displays the figure on the screen.

C. It clears the current figure.

D. It creates a new figure window.

Solution

1. Step 1: Understand the purpose of plt.savefig()

   This function is used to save the current plot or figure to a file, such as PNG or PDF.

2. Step 2: Differentiate from display functions

   Functions like plt.show() display the figure, but plt.savefig() writes it to disk.

3. Final Answer:

   It saves the current figure to a file on your computer. -> Option A

4. Quick Check:

   plt.savefig() saves figure [OK]

Hint: Remember: savefig saves, show displays [OK]

Common Mistakes:

• Confusing plt.savefig() with plt.show()
• Thinking savefig displays the plot
• Assuming savefig clears the figure

2. Which of the following is the correct syntax to save a figure as a PNG file named 'plot.png'?

easy

A. plt.save('plot.png')

B. plt.savefig('plot.png')

C. plt.savefig(plot.png)

D. plt.save_figure('plot.png')

Solution

1. Step 1: Recall the correct function name

   The function to save figures is plt.savefig(), not plt.save() or others.

2. Step 2: Check the argument format

   The filename must be a string, so it should be in quotes: 'plot.png'.

3. Final Answer:

   plt.savefig('plot.png') -> Option B

4. Quick Check:

   Correct function and string filename [OK]

Hint: Use plt.savefig('filename.ext') with quotes [OK]

Common Mistakes:

• Missing quotes around filename
• Using incorrect function name
• Passing filename without quotes

3. What will happen if you run this code?

import matplotlib.pyplot as plt
plt.plot([1, 2, 3], [4, 5, 6])
plt.savefig('myplot.pdf', dpi=300)
plt.close()

medium

A. The plot is shown on screen but not saved.

B. An error occurs because dpi is not allowed with PDF files.

C. A PDF file named 'myplot.pdf' is saved with high resolution.

D. The file 'myplot.pdf' is saved but with default dpi 72.

Solution

1. Step 1: Understand plt.savefig() with dpi parameter

   Setting dpi=300 increases the resolution of the saved image, including PDF files.

2. Step 2: Confirm file saving and closing

   The code saves the plot as 'myplot.pdf' with dpi 300, then closes the figure to free memory.

3. Final Answer:

   A PDF file named 'myplot.pdf' is saved with high resolution. -> Option C

4. Quick Check:

   savefig with dpi=300 saves high-res PDF [OK]

Hint: dpi controls resolution; works with PDF too [OK]

Common Mistakes:

• Thinking dpi is ignored for PDF
• Expecting plot to display automatically
• Confusing plt.close() with plt.show()

4. Identify the error in this code snippet that tries to save a figure:

import matplotlib.pyplot as plt
plt.plot([1, 2, 3], [4, 5, 6])
plt.savefig(myplot.png)
plt.show()

medium

A. The file extension '.png' is invalid.

B. plt.show() must come before plt.savefig().

C. plt.plot() is missing a figure creation step.

D. The filename is not enclosed in quotes.

Solution

1. Step 1: Check the filename argument in plt.savefig()

   The filename must be a string, so it needs quotes: 'myplot.png'.

2. Step 2: Verify order of plt.show() and plt.savefig()

   Calling plt.savefig() before plt.show() is correct and common practice.

3. Final Answer:

   The filename is not enclosed in quotes. -> Option D

4. Quick Check:

   Filename must be a string in quotes [OK]

Hint: Always put filename in quotes in savefig() [OK]

Common Mistakes:

• Omitting quotes around filename
• Thinking plt.show() must come first
• Assuming file extension is wrong

5. You want to save a plot tightly cropped around the content to avoid extra white space. Which plt.savefig() parameter helps achieve this?

hard

A. bbox_inches='tight'

B. format='png'

C. transparent=True

D. dpi=300

Solution

1. Step 1: Understand the role of bbox_inches

   The parameter bbox_inches='tight' trims the saved figure to fit the content closely, removing extra margins.

2. Step 2: Differentiate from other parameters

   dpi controls resolution, transparent controls background, and format sets file type, none affect cropping.

3. Final Answer:

   bbox_inches='tight' -> Option A

4. Quick Check:

   Use bbox_inches='tight' to crop figure [OK]

Hint: Use bbox_inches='tight' to crop saved figure [OK]

Common Mistakes:

• Confusing dpi with cropping
• Using transparent instead of bbox_inches
• Setting format does not crop figure