Label individual panels in a multi-panel ggplot2 - r

Question

I'm interested in trying to create simple corner labels for a multipanel figure I am preparing in ggplot. This is similar to this previously asked question, but the answers only explained how to include a label at the top of the plot, not produce a corner label in the format required by many journals. I hope to replicate something similar to the plotrix function corner.label() in ggplot2.
Here is an example using plottrix of what I would like to recreate in ggplot2.
require(plotrix)
foo1<-rnorm(50,25,5)
foo2<-rpois(50,25)
foo3<-rbinom(50,25,0.5)
foo4<-rnbinom(50,25,0.5)
par(mfrow=c(2,2))
hist(foo1)
corner.label(label='a',figcorner=T)
hist(foo2)
corner.label(label='b',figcorner=T)
hist(foo3)
corner.label(label='c',figcorner=T)
hist(foo4)
corner.label(label='d',figcorner=T)
This produces the following:
Thanks for any help in advance!

Answer 1

Two recent changes have made this a lot easier:
The latest release of ggplot2 has added the tag caption which can be used to label subplots.
The package patchwork makes it really easy to plot multiple ggplot objects. https://github.com/thomasp85/patchwork
This means that no altering of grobs is required. Adapting the reproducible example provided by Kev:
library(ggplot2)
# install.package("patchwork")
library(patchwork)
a <- 1:20
b <- sample(a, 20)
c <- sample(b, 20)
d <- sample(c, 20)
mydata <- data.frame(a, b, c, d)
myplot1 <- ggplot(mydata, aes(x=a, y=b)) + geom_point() + labs(tag = "A")
myplot2 <- ggplot(mydata, aes(x=b, y=c)) + geom_point() + labs(tag = "B")
myplot3 <- ggplot(mydata, aes(x=c, y=d)) + geom_point() + labs(tag = "C")
myplot4 <- ggplot(mydata, aes(x=d, y=a)) + geom_point() + labs(tag = "D")
myplot1 + myplot2 + myplot3 + myplot4
Extension: Changing Style:
If you want to change the labelling style, you can either set this individually for each plot or set a theme default. I would recommend the second approach. Add the following line before you build your plots to make the font bold and blue
ggplot2::theme_update(plot.tag = element_text(face = "bold", colour = "blue"))
For more information on customising the theme of ggplot2, see here.

Answer 2

I had the same problem and came up with the following solution, which is a bit different:
loading r packages
library(ggplot2)
library(grid)
library(gridExtra)
example data
a <- 1:20
b <- sample(a, 20)
c <- sample(b, 20)
d <- sample(c, 20)
create a data frame
mydata <- data.frame(a, b, c, d)
create example plots
myplot1 <- ggplot(mydata, aes(x=a, y=b)) + geom_point()
myplot2 <- ggplot(mydata, aes(x=b, y=c)) + geom_point()
myplot3 <- ggplot(mydata, aes(x=c, y=d)) + geom_point()
myplot4 <- ggplot(mydata, aes(x=d, y=a)) + geom_point()
set corner labels
myplot1 <- arrangeGrob(myplot1, top = textGrob("A", x = unit(0, "npc")
, y = unit(1, "npc"), just=c("left","top"),
gp=gpar(col="black", fontsize=18, fontfamily="Times Roman")))
myplot2 <- arrangeGrob(myplot2, top = textGrob("B", x = unit(0, "npc")
, y = unit(1, "npc"), just=c("left","top"),
gp=gpar(col="black", fontsize=18, fontfamily="Times Roman")))
myplot3 <- arrangeGrob(myplot3, top = textGrob("C", x = unit(0, "npc")
, y = unit(1, "npc"), just=c("left","top"),
gp=gpar(col="black", fontsize=18, fontfamily="Times Roman")))
myplot4 <- arrangeGrob(myplot4, top = textGrob("D", x = unit(0, "npc")
, y = unit(1, "npc"), just=c("left","top"),
gp=gpar(col="black", fontsize=18, fontfamily="Times Roman")))
plotting all plots on one page
grid.arrange(myplot1, myplot2, myplot3, myplot4, ncol = 2)

Answer 3

An example:
d <- data.frame(x = runif(16),
y = runif(16),
grp = rep(letters[1:4],each = 4))
ggplot(d,aes(x = x,y = y)) +
facet_wrap(~grp) +
geom_point() +
theme(strip.text = element_text(hjust = -0.05),
strip.background = element_blank())

Answer 4

Here's a solution using a custom labeller function. This doesn't invovle any manipulations to the data. Currently it only works with 1-dimensional facets (facet_wrap). I'm still working on how to increment along a 2-D grid...
Define the labeller function
make_labelstring <- function(mypanels) {
mylabels <- sapply(mypanels,
function(x) {LETTERS[which(mypanels == x)]})
return(mylabels)
}
label_panels <- ggplot2::as_labeller(make_labelstring)
Pass label_panels as the labeller to facet_wrap
library(ggplot2)
data("diamonds")
# create a faceted plot
ggplot(data = diamonds, aes(x = depth, y = price)) +
geom_point() +
facet_wrap(~cut, labeller = label_panels) +
theme(strip.text = element_text(hjust = -0),
strip.background = element_blank())