Creating graphs in a loop: one graph different from the others

Question

I have an elaborate code to create a series of graphs. I would like to put a vertical line in one of the many graphs I create.

Consider the following simple code:

library(ggplot2)
library(grid)
library(gridExtra)


plots <- list()

for (i in 1:4) {
  V1 <- rnorm(1000)
  V2 <- seq(1000)
  df <- data.frame(V1, V2)

plots[[i]] <- ggplot(df, aes(x= V2, y=V1)) +
    geom_point()+
    geom_vline(xintercept = 500, color="red")
}


grid.arrange(grobs=plots, nrow=2)

I would like to have the red vertical line for graph 4 but not the others. How would I do this efficiently?

Answer 1

just split your plot production and set a condition :)

library(ggplot2)
library(grid)
library(gridExtra)


plots <- list()

for (i in 1:4) {
    V1 <- rnorm(1000)
    V2 <- seq(1000)
    df <- data.frame(V1, V2)
    
    plots[[i]] <- ggplot(df, aes(x= V2, y=V1)) +
        geom_point()
    if (i == 4) plots[[i]] <- plots[[i]] + geom_vline(xintercept = 500, color="red")
}


grid.arrange(grobs=plots, nrow=2)