You want to add a text annotation to a plot.

Use annotate() and a text geom (Figure 7-1):

p <- ggplot(faithful, aes(x=eruptions, y=waiting)) + geom_point()

p + annotate("text", x=3, y=48, label="Group 1") +
    annotate("text", x=4.5, y=66, label="Group 2")

Figure 7-1. Text annotations

The annotate() function can be used to add any type of geometric object. In this case, we used geom="text".

Other text properties can be specified, as shown in Figure 7-2:

p + annotate("text", x=3, y=48, label="Group 1", family="serif",
             fontface="italic", colour="darkred", size=3) +
    annotate("text", x=4.5, y=66, label="Group 2", family="serif",
             fontface="italic", colour="darkred", size=3)