if else condition in ggplot to add an extra layer

This was done using ggplot2 2.1.0. I think you can do exactly what the OP wished, just by switching the parenthesis so that they encompass the entire if statement.

Here is an example that add a horizontal line depending on if Swtich is T or F. First, where the condition is TRUE

library(ggplot2)

df<-data.frame(x=1:10,y=11:20)
Switch=T

ggplot(df,aes(x,y))+
{if(Switch)geom_hline(yintercept=15)}+
  geom_point()

Now, the same thing but the condition is FALSE

df<-data.frame(x=1:10,y=11:20)
Switch=F

ggplot(df,aes(x,y))+
{if(Switch)geom_hline(yintercept=15)}+
  geom_point()