How to add titles between plots - r

Question

For the sake of simplicity, let's assume I have four graphs:
data("midwest", package = "ggplot2")
p1<-ggplot(midwest, aes(x=area, y=poptotal)) + geom_point()
p2<-ggplot(midwest, aes(x=area, y=poptotal)) + geom_point()
p3<-ggplot(midwest, aes(x=area, y=poptotal)) + geom_point()
p4<-ggplot(midwest, aes(x=area, y=poptotal)) + geom_point()
grid.arrange(p1,p2,p3,p4,ncol=2)
Now, I want to create a title (TITLE 1, TITLE 2) between each two titles,, as presented below:
Any ideas how to do it?

Answer 1

Here is a gtable solution to your problem. There might be easier solutions out there, but this should work.
First we'll bake in some titles in the leftmost plots
library(grid) # needed later for plotting
data("midwest", package = "ggplot2")
p1<-ggplot(midwest, aes(x=area, y=poptotal)) + geom_point() + ggtitle("Title 1")
p2<-ggplot(midwest, aes(x=area, y=poptotal)) + geom_point()
p3<-ggplot(midwest, aes(x=area, y=poptotal)) + geom_point() + ggtitle("Title 2")
p4<-ggplot(midwest, aes(x=area, y=poptotal)) + geom_point()
Then we can cbind and rbind the plots together as we see fit.
p12 <- cbind(ggplotGrob(p1), ggplotGrob(p2), size = "first")
p34 <- cbind(ggplotGrob(p3), ggplotGrob(p4), size = "first")
all <- rbind(p12, p34, size = "first")
grid.newpage(); grid.draw(all)
Note that we'd have to work with grid.newpage() and grid.draw() to get our plots, since we've left the ggplot sphere and are now in the realm of gtables and grid. Anyway, resulting plot looks like the following:
From your example I expect that you want these titles to be centered. This will be a bit more finicky:
# Decide what is a title
is_title <- grep("^title$", all$layout$name)
# Grab all titles
titles <- all$grobs[is_title]
# Exclude empty titles
is_title <- is_title[!sapply(titles, inherits, "zeroGrob")]
# Center title
all$grobs[is_title] <- lapply(all$grobs[is_title], function(title) {
title$children[[1]]$hjust <- 0.5
title$children[[1]]$x <- unit(0.5, "npc")
title
})
# Spread title over all panels
# You can see the number you'd need from the l/r coordinates of the 'panel' grobs
# which you can find by printing `all` or `all$layout`.
all$layout[is_title, "r"] <- 14
grid.newpage(); grid.draw(all)
EDIT: added example for adding extra titles
You can add extra titles, but you would need the gtable package for this.
library(gtable)
# First make extra titles
left <- textGrob("Left Title", gp = gpar(fontsize = 13.2, col = "black",
lineheight = 0.9, font = 1))
right <- textGrob("Right Title", gp = gpar(fontsize = 13.2, col = "black",
lineheight = 0.9, font = 1))
# Find a height that was 0, assign new height based on extra title
all$heights[[2]] <- unit(1, "grobheight", left)
# Add the titles (t = top position, l = left position)
all <- gtable_add_grob(all, left, t = 2, l = 5, clip = "off")
all <- gtable_add_grob(all, right, t = 2, l = 14, clip = "off")
grid.newpage(); grid.draw(all)