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.