How to mark certain point in ggpairs?

Here's minimal working problem. Consider the following data frame

DF <- data.frame(
  y=1:10,
  x=10:1,
  z=rep(5,10),
  a=11:20
)

Assume y is my response. Now, for every pair plot (y,_) in ggpairs(DF) I want to mark point 1, 5 and 10 with a different color. How do I do this?

There doesn't seem to be an in-built way of doing this in ggpairs. There is a way round it, which is to modify the plot it produces. Of necessity, this is a "hacky" solution. It first requires you to specify the points panels (i.e. the ones you wish to modify), and of course the rows whose points you wish to be red:

point_panels <- c(5, 9, 10, 11, 13:15)
red_points <- c(1, 5, 10)

If you have done that correctly, the following code should transform your plots appropriately:

ggp <- ggpairs(DF)
ggp$data$colors <- "normal"
ggp$data$colors[red_points] <- "special"

for(i in point_panels) {
  ggp$plots[[i]]$mapping <- 
    `class<-`(c(ggp$plots[[i]]$mapping, aes(color = colors)), "uneval")
}

ggp <- ggp + scale_color_manual(values = c("black", "red"))

So now when we do:

ggp

we get

If you want to add a legend, do

ggp$legend <- point_panels[1]
ggp