ggplot syntax confusion

[hellyj]

Aloha. I find that ggplot syntax is not consistent when used in a function versus interactively invoked. Maybe someone can comment on this quirk?

This syntax works inside a function:
p1 = ggplot(data, aes(x, y)) + geom_point()
invisible(print(p1))

This syntax does not work inside a function:
p1 = ggplot(data, aes(x, y))
p1 + geom_point()
invisible(print(p1))

The second syntax does work when interactively invoked. Why the difference, please?

[technocrat]

library(ggplot2)
library(patchwork)

p1 <- ggplot(mtcars, aes(mpg,hp)) + geom_point()

# these are equivalent
p1 / print(p1) / invisible(print(p1))

p <- function(x) {
  o = ggplot(x, aes(mpg,hp)) + geom_point()
  o / print(o) / invisible(print(o))
}

p(mtcars)

^{Created on 2023-05-22 with reprex v2.0.2}

[hellyj]

Thanks for the reply but the issue is not the print part. It's that

p1 + geom_point()

does not print any data on the p1 background when used this way in a function in a package I'm building. It does work when I execute the code interactively. However, it does work when geom_point is tacked on to the ggplot statement directly ( ggplot() + geom_point() ). I assume it has something to do with aes() but I can't find out what or why.

[technocrat]

Without a a reprex (see the FAQ), it's hard to see what's going on in your function.

library(ggplot2)

p <- ggplot(mtcars, aes(mpg,hp)) 

add_points <- function(x) {
  x + geom_point()
}

add_points(p)

[hellyj]

I know. I'll work on coming up with something but the environment is a bit complex so I think it somehow is data dependent. I haven't come up with a good example and the trivial tests I've tried all work as you've exhibited above.

[technocrat]

It doesn't have to be complete data, or even real data—just enough to illustrate the problem.

[hellyj]

Understand. l'll see if I can get something reproducible but minimal.

[hellyj]

However, it seems like the p1 + geom_point() is not executing when it is on a line by itself. I know this makes no sense but that's what it seems like.

[Matthias]

It is executed, but it's not saved.
you should use:
p1 = p1 + geom_point() to overwrite p1, so it can be recalled in the next step. You could also save it as p2 and then use print(p2).

[technocrat]

With

p2 <- p1 + geom_point()

there’s no need for print()

With a f() that creates a ggplot object, must take care to capture it

p3 <- f(…)

[Matthias]

hmm, okay that's true.
Still the invisible(print(p1)) statement in the next line overwrites the correct plot with the unfinished one as long as p1 doesn't include the added geom_point().