Enriching a ggplot2 plot with multiple geom_segment in a loop?

Question

I successfully create a plot using the following:

# suppose I have a p <- ggplot(data=df, ...) then the following works 
# I get those two segments plotted correctly
p <- p + geom_segment(aes(x=1,y=103,xend=1,yend=107))
p <- p + geom_segment(aes(x=5,y=103,xend=5,yend=107))

However if I do:

values <- c(1, 5)
for (i in values) {
   p <- p + geom_segment(aes(x=i,y=103,xend=i,yend=107))
}

It doesn't work, only the last segment is created. Can anyone advice what's wrong here?

Answer 1

It has to do with the lazy evaluation of the aes() values. You are binding to the variable i but not actually doing anything with it in the loop. The mappings aren't resolved till you actually print(p). Essentially this means they are all being bound to i and after the loop exits, i will have the value it had during the final loop.

So the problem really is you shounld't be using aes() here as you don't really want active binding. Just set the x and xend values outside the aes(). (And since the y's are constant they should be outside the aes() as well).

values <- c(1, 5)
for (i in values) {
   p <- p + geom_segment(x=i, y=103, xend=i, yend=107)
}

Answer 2

An alternative approach would be to avoid using a loop at all. You can pack your segment data up in a separate data.frame from your main data and use aes() to plot everything at once like so:

segment_data = data.frame(
    x = c(1, 5),
    xend = c(1, 5), 
    y = c(103, 103),
    yend = c(107, 107)
)

p = ggplot(df, ...) +
geom_segment(data = segment_data, aes(x = x, y = y, xend = xend, yend = yend))