Saving plots (ggsave) in R Programming - Mini Project: Build & Apply

Saving plots with ggsave in R

📖 Scenario: You have created a simple plot in R using ggplot2. Now, you want to save this plot as an image file so you can share it or use it later.

🎯 Goal: Learn how to save a plot using the ggsave function in R.

📋 What You'll Learn

Create a basic plot using ggplot2

Set a filename to save the plot

Use ggsave to save the plot as a PNG file

Print a confirmation message after saving

💡 Why This Matters

🌍 Real World

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

💼 Career

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

1

Create a simple plot using ggplot2

Load the ggplot2 library and create a plot called my_plot using the built-in mtcars dataset. Plot mpg on the y-axis and wt on the x-axis with points.

R Programming

# Load ggplot2 and create a plot called my_plot
# Your code here

Hint

Use library(ggplot2) to load the package. Then assign the plot to my_plot.

2

Set the filename to save the plot

Create a variable called filename and set it to the string "my_mtcars_plot.png".

R Programming

library(ggplot2)
my_plot <- ggplot(mtcars, aes(x = wt, y = mpg)) + geom_point()
# Create a variable filename with the value "my_mtcars_plot.png"
# Your code here

Hint

Use filename <- "my_mtcars_plot.png" to set the file name.

3

Save the plot using ggsave

Use the ggsave function to save my_plot to the file named filename. Use default width and height.

R Programming

library(ggplot2)
my_plot <- ggplot(mtcars, aes(x = wt, y = mpg)) + geom_point()
filename <- "my_mtcars_plot.png"
# Save the plot using ggsave
# Your code here

Hint

Call ggsave(filename, plot = my_plot) to save the plot.

4

Print confirmation message

Print the message "Plot saved as my_mtcars_plot.png" to confirm the plot was saved.

R Programming

library(ggplot2)
my_plot <- ggplot(mtcars, aes(x = wt, y = mpg)) + geom_point()
filename <- "my_mtcars_plot.png"
ggsave(filename, plot = my_plot)
# Print confirmation message
# Your code here

Hint

Use print("Plot saved as my_mtcars_plot.png") to show the message.

Practice

1. What does the ggsave() function do in R's ggplot2 package?

easy

A. Saves the current or specified plot as an image file

B. Displays the plot on the screen

C. Creates a new plot object

D. Deletes the last plot created

Solution

1. Step 1: Understand the purpose of ggsave()

   The ggsave() function is designed to save plots created by ggplot2 as image files like PNG, PDF, or JPEG.

2. Step 2: Differentiate from other plot functions

   Unlike functions that display or create plots, ggsave() specifically saves the plot to a file on disk.

3. Final Answer:

   Saves the current or specified plot as an image file -> Option A

4. Quick Check:

   ggsave() = Save plot [OK]

Hint: Remember: ggsave saves plots to files, not just shows them [OK]

Common Mistakes:

• Thinking ggsave displays the plot
• Confusing ggsave with plot creation
• Assuming ggsave deletes plots

2. Which of the following is the correct syntax to save the last plot as a PNG file named "plot.png" with default size using ggsave()?

easy

A. ggsave("plot.png", plot)

B. ggsave(plot.png)

C. ggsave("plot.png")

D. ggsave(plot, "plot.png")

Solution

1. Step 1: Check the basic ggsave syntax

   The simplest way to save the last plot is to call ggsave() with the filename as a string, e.g., ggsave("plot.png").

2. Step 2: Identify incorrect syntax

   ggsave(plot.png) lacks quotes around filename, causing an error. Options A and D reference an undefined plot or have wrong argument order.

3. Final Answer:

   ggsave("plot.png") -> Option C

4. Quick Check:

   Filename as string = ggsave("plot.png") [OK]

Hint: Filename must be a quoted string in ggsave() [OK]

Common Mistakes:

• Forgetting quotes around filename
• Passing plot object without naming argument
• Swapping argument order

3. What will be the result of this code?

library(ggplot2)
p <- ggplot(mtcars, aes(x=wt, y=mpg)) + geom_point()
ggsave("myplot.pdf", plot = p, width = 5, height = 4)

medium

A. Saves the plot 'p' as a 5x4 inch PDF file named 'myplot.pdf'

B. Throws an error because width and height are missing units

C. Saves the last plot instead of 'p' ignoring the plot argument

D. Creates a PNG file instead of PDF

Solution

1. Step 1: Analyze the ggsave parameters

   The code specifies filename "myplot.pdf", plot object p, and size 5 by 4 inches. This is valid syntax.

2. Step 2: Understand default units and file type

   Width and height default to inches if units not specified. The file extension '.pdf' sets the output format to PDF.

3. Final Answer:

   Saves the plot 'p' as a 5x4 inch PDF file named 'myplot.pdf' -> Option A

4. Quick Check:

   ggsave() saves plot with given size and format [OK]

Hint: File extension sets format; width/height default to inches [OK]

Common Mistakes:

• Assuming width/height need units explicitly
• Thinking plot argument is ignored
• Confusing file format with extension

4. Identify the error in this code snippet:

library(ggplot2)
p <- ggplot(mtcars, aes(x=wt, y=mpg)) + geom_point()
ggsave(filename = myplot.png, plot = p)

medium

A. Missing library call for ggsave

B. Plot object 'p' is not defined

C. ggsave() cannot save PNG files

D. Missing quotes around filename 'myplot.png'

Solution

1. Step 1: Check the filename argument

   The filename argument must be a string in quotes. Here, myplot.png is unquoted, causing an error.

2. Step 2: Verify other parts

   The plot object p is defined, PNG is supported, and library(ggplot2) is loaded, so no other errors.

3. Final Answer:

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

4. Quick Check:

   Filename must be quoted string [OK]

Hint: Always put quotes around filenames in ggsave() [OK]

Common Mistakes:

• Forgetting quotes around filename
• Assuming ggsave needs extra libraries
• Thinking PNG format is unsupported

5. You want to save two different ggplot objects, p1 and p2, as PNG files with different sizes. Which code correctly saves p1 as 6x4 inches and p2 as 8x6 inches?

hard

A. ggsave(p1, "p1.png", width = 6, height = 4)
ggsave(p2, "p2.png", width = 8, height = 6)

B. ggsave("p1.png", plot = p1, width = 6, height = 4)
ggsave("p2.png", plot = p2, width = 8, height = 6)

C. ggsave("p1.png", p1, 6, 4)
ggsave("p2.png", p2, 8, 6)

D. ggsave("p1.png", width = 6, height = 4)
ggsave("p2.png", width = 8, height = 6)

Solution

1. Step 1: Check correct argument order and naming

   ggsave("p1.png", plot = p1, width = 6, height = 4) ggsave("p2.png", plot = p2, width = 8, height = 6) uses correct syntax: filename as first argument (string), plot named with plot=, and width/height named.

2. Step 2: Identify errors in other options

   ggsave(p1, "p1.png", width = 6, height = 4) ggsave(p2, "p2.png", width = 8, height = 6) swaps plot and filename positionally without naming plot=, causing plot object to be assigned to filename (error). ggsave("p1.png", p1, 6, 4) ggsave("p2.png", p2, 8, 6) passes sizes positionally to wrong parameters (device/path). ggsave("p1.png", width = 6, height = 4) ggsave("p2.png", width = 8, height = 6) omits plot argument, so last plot saved instead.

3. Final Answer:

   Correctly saves both plots with specified sizes -> Option B

4. Quick Check:

   Filename first, plot named, size named = ggsave("p1.png", plot = p1, width = 6, height = 4) ggsave("p2.png", plot = p2, width = 8, height = 6) [OK]

Hint: Always name plot= and specify filename as first argument [OK]

Common Mistakes:

• Swapping filename and plot arguments
• Not naming plot argument
• Omitting plot argument saves last plot instead