You want to add annotations to each facet in a plot.

Create a new data frame with the faceting variable(s), and a value to use in each facet. Then use geom_text() with the new data frame (Figure 7-17):

# The base plot
p <- ggplot(mpg, aes(x=displ, y=hwy)) + geom_point() + facet_grid(. ~ drv)

# A data frame with labels for each facet
f_labels <- data.frame(drv = c("4", "f", "r"), label = c("4wd", "Front", "Rear"))

p + geom_text(x=6, y=40, aes(label=label), data=f_labels)

# If you use annotate(), the label will appear in all facets
p + annotate("text", x=6, y=42, label="label text")

Figure 7-17. Top: different annotations in each facet; bottom: the same annotation in each facet