How to add labels to multiple ggplot graphs (A, B, C) - r

Question

I am trying to add the labels A, B, and C to the top left hand corner of each of these graphs. I have tried cowplot::draw_plot_label(), but nothing seems to work. Can anyone help?
These A, B and C labels are not the main title of each plot.
# Packages
library(ggplot2)
library(gridExtra)
library(cowplot)
# 1st plot
p1 <- ggplot(data = new_data %>%
filter(Species =="Sharksucker_Remora")) +
scale_colour_manual(values=c(Sharksucker_Remora="black"), labels = c("Sharksucker Remora")) +
geom_line(mapping = aes(x = Date, y = Proportion, group = Species, colour = Species)) +
xlab("") +
ylab("Proportion") +
theme(legend.position="top") +
theme(axis.text.x = element_text(angle = 90, vjust = 0.5, hjust = 1)) + labs(colour = ~italic(M.alfredi)~"Hitchhiker Species:") +
theme(legend.key=element_blank())
# 2nd plot
p2 <- ggplot(data = new_data %>%
filter(Species !="Sharksucker_Remora")) +
geom_line(mapping = aes(x = Date, y = Proportion, group = Species, colour = Species)) +
scale_colour_manual(values=c(Golden_Trevally="goldenrod2", Red_Snapper="firebrick2", Juvenile_Remora="darkolivegreen3"), labels = c("Juvenile Remora", "Golden Trevally", "Red Snapper")) +
xlab("") + ylab("Proportion") + labs(colour = "") + theme(legend.position="top") + theme(legend.key=element_blank()) +
theme(axis.text.x = element_text(angle = 90, vjust = 0.5, hjust = 1))
# 3rd plot
p3 <- ggplot(data = new_data_counts) +
geom_bar(mapping = aes(x = Date, y = Count), stat =
'identity') +
xlab("Date (2015-2019)") + ylab("Total"~italic
(M.alfredi)~"Sightings") +
draw_plot_label(label =c("C") + theme(axis.text.x =
element_text(angle = 90, vjust = 0.5, hjust = 1))
# The grid
grid.arrange(p1,p2,p3)

Answer 1

I suggest you use labs(..., tag = ...) and theme(plot.tag = element_text()).
The code show how you can format the main title (here centered with hjust = 0.5) and the tag inside the theme() function. See the reproducible example, below:
# Packages
library(ggplot2)
library(gridExtra)
# library(cowplot) # not necessary here
# Plots
p1 <- ggplot() +
labs(title = "plot 1", tag = "A") +
theme(plot.title = element_text(hjust = 0.5),
plot.tag = element_text())
p2 <- ggplot() +
labs(title = "plot 2", tag = "B") +
theme(plot.title = element_text(hjust = 0.5),
plot.tag = element_text())
grid.arrange(p1, p2)
If you want the tag (A, B, C) to be inside the plotting area, this post suggest to use plot.tag.position = c(x, y). See for example:
p3 <- ggplot() +
labs(title = "plot 3", tag = "C") +
theme(plot.title = element_text(hjust = 0.5),
plot.tag = element_text(),
plot.tag.position = c(0.1, 0.8))
p3

Answer 2

Have you tried the package egg?
https://cran.r-project.org/web/packages/egg/vignettes/Overview.html
library(tidyverse)
library(magrittr)
data <- list()
for(i in 1:6) data[[i]] <- rnorm(100,0,1)
data %<>% bind_cols() %>% setNames(paste0("var",1:6))
p1 <- ggplot(data,aes(x = var1, y = var2)) + geom_point()
p2 <- ggplot(data,aes(x = var3, y = var4)) + geom_point()
p3 <- ggplot(data,aes(x = var5, y = var6)) + geom_point()
egg::ggarrange(p1,p2,p3,ncol = 1,
labels = c("A","B","C"))

Answer 3

Another option is using the patchwork package with plot_annotation which has the tag_levels argument which gives the possibility to add tags like letters or numbers. First a reproducible example with letters:
library(patchwork)
library(ggplot2)
p1 <- ggplot(mtcars) +
geom_point(aes(hp, disp)) +
ggtitle('Plot 1')
p2 <- ggplot(mtcars) +
geom_boxplot(aes(gear, mpg, group = gear)) +
ggtitle('Plot 2')
p1 + p2 & plot_annotation(tag_levels = 'A')
Created on 2022-08-21 with reprex v2.0.2
Another option with numbers where you change the tag_levels to "1" like this:
p1 + p2 & plot_annotation(tag_levels = '1')
Created on 2022-08-21 with reprex v2.0.2
As you can see, the tags have letters or numbers. Check the links above for more information and options.