Saving figures in Data Analysis Python - Mini Project: Build & Apply

Saving figures

📖 Scenario: You have created a simple line plot showing sales data over a week. Now, you want to save this plot as an image file so you can share it or use it in reports.

🎯 Goal: Create a line plot using sales data and save the figure as a PNG file named sales_plot.png.

📋 What You'll Learn

Create a list called sales with the values: 150, 200, 250, 300, 280, 320, 400

Create a list called days with the values: 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun'

Use matplotlib to plot sales against days

Save the figure as sales_plot.png using matplotlib's savefig method

Print the message "Figure saved as sales_plot.png" after saving

💡 Why This Matters

🌍 Real World

Saving figures is important when you want to share your data visualizations with others or include them in reports and presentations.

💼 Career

Data scientists and analysts often save plots to communicate insights clearly and professionally.

1

Create sales and days data lists

Create a list called sales with these exact values: 150, 200, 250, 300, 280, 320, 400. Also create a list called days with these exact values: 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun'.

Data Analysis Python

# Create the sales and days lists
# Your code here

Hint

Use square brackets to create lists. Separate values with commas.

2

Import matplotlib and set up the plot

Import matplotlib.pyplot as plt. Then use plt.plot(days, sales) to create a line plot of sales over days.

Data Analysis Python

sales = [150, 200, 250, 300, 280, 320, 400]
days = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
# Import matplotlib.pyplot as plt
# Create a line plot of sales over days
# Your code here

Hint

Use import matplotlib.pyplot as plt to import. Then call plt.plot(days, sales).

3

Save the figure as a PNG file

Use plt.savefig('sales_plot.png') to save the current figure as a PNG file named sales_plot.png.

Data Analysis Python

import matplotlib.pyplot as plt

sales = [150, 200, 250, 300, 280, 320, 400]
days = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']

plt.plot(days, sales)
# Save the figure as 'sales_plot.png'
# Your code here

Hint

Use plt.savefig('sales_plot.png') to save the figure.

4

Print confirmation message

Write a print statement to display the message "Figure saved as sales_plot.png" after saving the figure.

Data Analysis Python

import matplotlib.pyplot as plt

sales = [150, 200, 250, 300, 280, 320, 400]
days = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']

plt.plot(days, sales)
plt.savefig('sales_plot.png')
# Print confirmation message
# Your code here

Hint

Use print("Figure saved as sales_plot.png") to show the message.

Practice

1. What is the main function used in Python's matplotlib to save a plot as an image file?

easy

A. plt.show()

B. plt.savefig()

C. plt.plot()

D. plt.save()

Solution

1. Step 1: Identify the function for saving plots

   The function plt.savefig() is designed to save the current figure to a file.

2. Step 2: Differentiate from other functions

   plt.show() displays the plot, plt.plot() creates the plot, and plt.save() does not exist.

3. Final Answer:

   plt.savefig() -> Option B

4. Quick Check:

   Save plot = plt.savefig() [OK]

Hint: Remember: save figure uses 'savefig' word [OK]

Common Mistakes:

• Confusing plt.show() with saving
• Using plt.save() which is not a valid function
• Thinking plt.plot() saves the figure

2. Which of the following is the correct syntax to save a plot as a PNG file with matplotlib?

easy

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

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

C. plt.savefig(plot.png)

D. plt.savefig('plot')

Solution

1. Step 1: Check the function name and argument type

   The correct function is plt.savefig() and it requires the filename as a string.

2. Step 2: Verify the filename format

   The filename must be a string with the extension, e.g., 'plot.png'. plt.savefig('plot.png') matches this exactly.

3. Final Answer:

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

4. Quick Check:

   Save with filename string = plt.savefig('plot.png') [OK]

Hint: Use quotes around filename with extension [OK]

Common Mistakes:

• Omitting quotes around filename
• Using plt.save() instead of plt.savefig()
• Missing file extension in filename

3. What will be the effect of this code snippet?

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

medium

A. Saves a PNG file named 'myplot.pdf' ignoring dpi and bbox_inches

B. Displays the plot on screen with 300 dpi resolution

C. Raises an error because dpi cannot be set for PDF files

D. Saves a PDF file named 'myplot.pdf' with high resolution and tight layout

Solution

1. Step 1: Understand plt.savefig parameters

   The code saves the plot as 'myplot.pdf' with dpi=300 for high resolution and bbox_inches='tight' to reduce whitespace.

2. Step 2: Confirm file format and parameters compatibility

   PDF supports dpi and bbox_inches parameters, so no error occurs and the file is saved correctly.

3. Final Answer:

   Saves a PDF file named 'myplot.pdf' with high resolution and tight layout -> Option D

4. Quick Check:

   Save PDF with dpi and tight bbox = correct [OK]

Hint: File extension sets format; dpi and bbox_inches improve quality [OK]

Common Mistakes:

• Thinking dpi is ignored for PDF
• Confusing save with display (plt.show())
• Assuming dpi causes error for PDF

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

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

medium

A. File extension '.png' is invalid

B. plt.plot() must be called after plt.savefig()

C. Missing quotes around filename 'myplot.png'

D. dpi parameter is required in plt.savefig()

Solution

1. Step 1: Check the argument to plt.savefig()

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

2. Step 2: Verify other parts of the code

   Calling plt.plot() before plt.savefig() is correct, dpi is optional, and '.png' is a valid extension.

3. Final Answer:

   Missing quotes around filename 'myplot.png' -> Option C

4. Quick Check:

   Filename must be string with quotes [OK]

Hint: Always put filename in quotes when saving [OK]

Common Mistakes:

• Forgetting quotes around filename
• Thinking dpi is mandatory
• Misordering plot and savefig calls

5. You want to save a plot as a high-quality PNG file with minimal white space around it. Which of the following code snippets achieves this correctly?

hard

A. plt.savefig('figure.png', dpi=600, bbox_inches='tight')

B. plt.savefig('figure.png', dpi=72, bbox_inches='standard')

C. plt.savefig('figure.png', dpi=600, bbox_inches='loose')

D. plt.savefig('figure.png', dpi='high', bbox_inches='tight')

Solution

1. Step 1: Understand dpi and bbox_inches parameters

   High dpi (e.g., 600) means better image quality. bbox_inches='tight' reduces extra white space.

2. Step 2: Evaluate each option

   plt.savefig('figure.png', dpi=600, bbox_inches='tight')

   uses dpi=600 and bbox_inches='tight' correctly.

   plt.savefig('figure.png', dpi=72, bbox_inches='standard')

   uses low dpi and invalid bbox_inches.

   plt.savefig('figure.png', dpi=600, bbox_inches='loose')

   uses bbox_inches='loose' which adds space.

   plt.savefig('figure.png', dpi='high', bbox_inches='tight')

   uses invalid dpi value 'high'.

3. Final Answer:

   plt.savefig('figure.png', dpi=600, bbox_inches='tight') -> Option A

4. Quick Check:

   High dpi + tight bbox = best quality [OK]

Hint: Use dpi=600 and bbox_inches='tight' for quality and layout [OK]

Common Mistakes:

• Using invalid dpi values like strings
• Choosing bbox_inches='loose' which adds whitespace
• Ignoring dpi leading to low-quality images