r - How can I combine multiple ggplot2 elements into the return of a function?

Question

If I try to manually compose some elements of a ggplot2 plot, it works just fine:

> p <- ggplot(aes(x = mpg, y = hp), data = mtcars)
> p + geom_vline(xintercept = 20) + geom_point(data = mtcars)

But if I try to bundle some of the composition into a function, I get an error:

> myFunction <- function() {
+   return(
+     geom_vline(xintercept = 20) + geom_point(data = mtcars)
+   )
+ }
> p <- ggplot(aes(x = mpg, y = hp), data = mtcars)
> p + myFunction()
Error in geom_vline(xintercept = 20) + geom_point(data = mtcars) : 
  non-numeric argument to binary operator

Am I missing something in ggplot2 notation for properly combining ggplot2 elements within a function body?

Answer 1

ggplot2 supports "list" of the elements:

myFunction <- function()
 list(geom_vline(xintercept = 20),
      geom_point(data = mtcars))

p <- ggplot(aes(x = mpg, y = hp), data = mtcars)
p + myFunction()

you can keep in a list any piece that ggplot2 function returns, including labs(), opts(), etc, and then use "+" for bind ggplot2 base layer and the piece in the list.

Probably this feature is not widely known , but is very useful when anyone want to re-use a piece of elements.