Saving plots (ggsave) in R Programming

Introduction

Saving plots lets you keep your graphs as image files. This helps you share or use them later.

You want to save a graph to include in a report or presentation.

You need to keep a copy of your plot for future reference.

You want to save a plot in a specific format like PNG or PDF.

You want to save multiple plots automatically in a script.

You want to control the size and resolution of your saved plot.

Syntax

R Programming

ggsave(filename, plot = last_plot(), device = NULL, path = NULL, scale = 1, width = NA, height = NA, units = c("in", "cm", "mm"), dpi = 300, limitsize = TRUE, ...)

filename is the name of the file you want to save, including extension like .png or .pdf.

If you don't specify plot, ggsave saves the last plot you made.

Examples

Saves the last plot as a PNG file named 'myplot.png' in the current folder.

R Programming

ggsave("myplot.png")

Saves the last plot as a PDF with width 5 inches and height 4 inches.

R Programming

ggsave("plot.pdf", width = 5, height = 4, units = "in")

Saves a specific plot object my_plot as a PNG with high resolution (600 dpi).

R Programming

ggsave("plot2.png", plot = my_plot, dpi = 600)

Sample Program

This program makes a scatter plot of car weight vs. miles per gallon. Then it saves the plot as a PNG file named 'car_plot.png' with size 6x4 inches and 300 dpi resolution.

R Programming

library(ggplot2)

# Create a simple plot
p <- ggplot(mtcars, aes(x = wt, y = mpg)) + geom_point()

# Save the plot as a PNG file
# This saves 'car_plot.png' in your working directory

ggsave("car_plot.png", plot = p, width = 6, height = 4, units = "in", dpi = 300)

print("Plot saved as car_plot.png")

Important Notes

Make sure your working directory is where you want to save the file, or provide a full path in filename.

You can save plots in many formats like PNG, PDF, JPEG, TIFF by changing the file extension.

If you get an error about size, try setting limitsize = FALSE to allow bigger images.

Summary

ggsave saves your ggplot graphs as image files.

You can control file name, format, size, and resolution easily.

It saves the last plot by default or a specific plot if you provide one.

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