Saving plots (ggsave) in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the ggsave() function in R?

ggsave() saves a plot created with ggplot2 to a file on your computer, like a PNG or PDF.

beginner

Which argument in ggsave() specifies the filename and file type?

The filename argument sets the name and extension (like "plot.png") to decide the file type.

beginner

How do you save the last plot you made using ggsave() without specifying the plot object?

If you don't give a plot to ggsave(), it saves the last plot you displayed.

intermediate

What arguments control the size of the saved plot in ggsave()?

width and height set the size of the saved image, usually in inches.

intermediate

Can ggsave() save plots in formats other than PNG? Name two.

Yes! It can save as PDF, JPEG, TIFF, SVG, and more, depending on the file extension you use.

What happens if you call ggsave("myplot.pdf") without specifying a plot?

A It opens a dialog to select a plot.

B It throws an error because no plot is specified.

C It saves an empty file.

D It saves the last plot you created.

If no plot is given, ggsave() saves the last plot displayed.

Which argument do you use to change the width of the saved plot in ggsave()?

A size

B width

C length

D scale

The width argument sets the width of the saved image.

If you want to save a plot as a PNG file, what should the filename end with?

A .png

B .jpeg

C .pdf

D .txt

The filename should end with .png to save as a PNG image.

Which package must be loaded to use ggsave()?

A dplyr

B shiny

C ggplot2

D tidyr

ggsave() is part of the ggplot2 package.

What does ggsave("plot.svg", plot = myplot) do?

A Saves the plot object <code>myplot</code> as an SVG file.

B Saves the last plot as SVG.

C Saves the plot as a PNG file.

D Throws an error because SVG is not supported.

This saves the plot stored in myplot as an SVG file named "plot.svg".

Explain how to save a ggplot as a PDF file with a custom width and height using ggsave().

Think about the filename and size arguments.

Describe what happens if you call ggsave() without specifying a plot and how you can save a specific plot instead.

Consider the default behavior and how to override it.

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