Add multiple curves / functions to one ggplot through looping

Question

I want to add multiple curves to one ggplot. With the normal plot and curve(add=TRUE) i can loop the curve function and can add multip

Answer 1

The issue is that all the stat_functions you are refer to the same i variable. And as a consequence, each function you add will overlay perfectly with the others.

The solution is to reassign the variable in a local scope, to make a local copy for each iteration:

p1 = ggplot(data.frame(x = 1 : 200)) + aes(x)

for (i in 1:10){
    p1 = local({
        j = i
        p1 + stat_function(fun = function(x) x + j * 3, col = j)
    })
}

To make matters more confusing, you don’t actually have to give the local variable a new name; you could just as easily have written i = i, and continued to use i. The reason is that this assignment will generate a new local variable i that masks the non-local variable i. I hope we can agree that writing such code is confusing, and a bad idea.

I’ve also taken the liberty of simplifying your code slightly by moving the data x out of the stat_function, and into the ggplot object directly.

However, it is altogether cleaner not to use a loop and reassignment here. Instead you can use lapply or map (from the purrr package):

p1 = ggplot(data.frame(x = 1 : 200)) +
    aes(x) +
    map(
        1 : 10,
        ~ stat_function(fun = function (x) x + .x * 3, color = .x)
    )

This is shorter, more readable (it focuses on the “what” rather than the “how”, i.e. the mechanics of the loop), and it uses a single assignment.

Answer 2

Try:

library(ggplot2)
add_curve <- function(plot, i) {
  return(plot + stat_function(aes(x=1:200),fun = function(x) {x+i*3}, col=i))
}

p1 <- ggplot()


for (i in 1:10){
  p1<- add_curve(p1, i)
}

p1

Output is:

Or alternatively you can also define your function inside the for-loop:

for (i in 1:10){
  add_curve <- function(plot, i) {
    return(plot + stat_function(aes(x=1:200),fun = function(x) {x+i*3}, col=i))
  }
  p1<- add_curve(p1, i)
}

Or as a (somewhat obscure) Reduce + lapply one-liner (thx to @KonradRudolph):

eval(
  Reduce(function(a, b) call('+', a, b), 
         lapply(1:10, function(i) {
           bquote(stat_function(aes(x=1:200),fun = function(x) {x+.(i)*3
             }, col=.(i)))}), init = quote(ggplot())))

The idea is to build the whole ggplot() call at once and then evaluate it.