Knowledge Quiz: Tutorial 3

Note

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• Click the question-mark button to see an explanation of the solution.

What is the key difference between facet_wrap(~x) and facet_grid(x ~ y)?

facet_wrap() changes the data being plotted; facet_grid() only changes cosmetic styling.facet_grid() can only be used with geom_bar().There is no meaningful difference; the two functions are interchangeable in every situation.facet_wrap() facets on one variable and reflows panels into a grid; facet_grid() facets on two variables and reserves a panel for every combination, even ones with no data.

• Wrong: neither faceting function changes which data is plotted; both only change how it’s split across panels.
• Wrong: facet_grid() works with any geometry, not just geom_bar().
• Wrong: the two produce different panel layouts (and facet_grid() reserves empty panels), so they are not interchangeable.
• Correct: this is exactly the distinction demonstrated with the mpg drv/cyl/class examples.

Which of the following statements about saving ggplot2 plots are TRUE? (Select all that apply.)

You must use pdf()/png()/jpeg() and dev.off(); ggsave() cannot save PDF or JPEG files.The file format ggsave() produces is determined by the extension in the file name (e.g., .png vs .pdf).Calling pdf("file.pdf") and then drawing a plot, without ever calling dev.off(), can result in a broken or empty file.ggsave() lets you specify width, height, and resolution directly, without manually opening and closing a graphics device.

• FALSE: ggsave() supports PDF, JPEG, PNG, and several other formats via the same file-extension mechanism.
• TRUE: ggsave() infers the output format from the file extension you provide.
• TRUE: forgetting dev.off() leaves the graphics device open and the file incomplete.
• TRUE: this is the main convenience ggsave() adds over the base R device functions.

You run p1 <- ggplot(mpg) + geom_bar(aes(x = class)) in an R script (not interactively at the console) and no plot appears. What is the best explanation?

ggplot2 plots can only be displayed inside grid.arrange().The code contains an error and R silently failed.geom_bar() cannot be assigned to a variable.Assigning a plot to a variable stores it but does not print/display it; you’d need to run p1 or print(p1) to see it.

• Wrong: grid.arrange() is only one way to display saved plots; printing a single saved plot works the same way as any other R object.
• Wrong: the code is valid; nothing has failed.
• Wrong: any ggplot2 object, including one built with geom_bar(), can be assigned to a variable.
• Correct: assignment (<-) stores the object silently, exactly like any other R object; a top-level expression (or an explicit print()) is needed to actually display it.