How to add labels to plot with facets?

I am attempting to add labels (capital letters) to each plot in the following facet_grid:

p <- ggplot(mpg, aes(displ, cty)) + geom_point()
p + facet_grid(drv ~ cyl)

How about this? Fixing the position of label as the upper-left corner of each plot panel:

p + facet_grid(drv ~ cyl)+ annotate('text', label = LETTERS[1:12], x=min(mpg$displ), y=max(mpg$cty))

You can also fine-tune the position of the label proportional to both axis by using:

x=mean(mpg$displ)*0.6, y=max(mpg$cty)*0.97

Load required packages and set the theme function theme_light() [ggplot2] as the default theme:

library(ggplot2)
theme_set(
  theme_light() + theme(legend.position = "top")
  )

Create a box plot filled by groups:

# Load data and convert dose to a factor variable
data("ToothGrowth")
ToothGrowth$dose <- as.factor(ToothGrowth$dose)

# Box plot, facet accordding to the variable dose and supp
p <- ggplot(ToothGrowth, aes(x = dose, y = len)) + 
  geom_boxplot(aes(fill = supp), position = position_dodge(0.9)) +
  scale_fill_viridis_d() 
p + facet_grid(dose ~ supp)

In the following R code, facets are labelled by combining the name of the grouping variable with group levels. The labeller function label_both is used.

p + facet_grid(dose ~ supp, labeller = label_both)

A simple way to modify facet label text, is to provide new labels as a named character vector:

# New facet label names for dose variable
dose.labs <- c("D0.5", "D1", "D2")
names(dose.labs) <- c("0.5", "1", "2")

# New facet label names for supp variable
supp.labs <- c("Orange Juice", "Vitamin C")
names(supp.labs) <- c("OJ", "VC")

# Create the plot
p + facet_grid(
  dose ~ supp, 
  labeller = labeller(dose = dose.labs, supp = supp.labs)
  )

An alternative solution to change the facet labels, is to modify the data:

df <- ToothGrowth
# Modify the data
df$dose <- factor(df$dose, levels = c("0.5", "1", "2"), 
                  labels = c("D0.5", "D1", "D2"))
df$supp <- factor(df$supp, levels = c("OJ", "VC"),
                  labels = c("Orange Juice", "Vitamin C")
                  )
# Create the plot
ggplot(df, aes(x = dose, y = len)) + 
  geom_boxplot(aes(fill = supp)) +
  facet_grid(dose ~ supp)