r - pass string to facet_grid : ggplot2

Question

In ggplot2 you can pass character arguments inside a user defined function using aes_string. How can you do the same for facet grid which takes a formula, not aes?

FUN <- function(data, x, y, fac1, fac2) {
     ggplot(data = data, aes_string(x=x, y=y)) +
     geom_point() + facet_grid(as.formula(substitute(fac1 ~ fac2)))
}


FUN(mtcars, 'hp', 'mpg', 'cyl', 'am')

Answer 1

reformulate() seems to work just fine.

FUN <- function(data, x, y, fac1, fac2) {
      ggplot(data = data, aes_string(x=x, y=y)) +
      geom_point() + facet_grid(reformulate(fac2,fac1))
}

FUN(mtcars, 'hp', 'mpg', 'cyl', 'am')