You want to add labels to the bars in a bar graph.

Add geom_text() to your graph. It requires a mapping for x, y, and the text itself. By setting vjust (the vertical justification), it is possible to move the text above or below the tops of the bars, as shown in Figure 3-22:

library(gcookbook) # For the data set

# Below the top
ggplot(cabbage_exp, aes(x=interaction(Date, Cultivar), y=Weight)) +
    geom_bar(stat="identity") +
    geom_text(aes(label=Weight), vjust=1.5, colour="white")

# Above the top
ggplot(cabbage_exp, aes(x=interaction(Date, Cultivar), y=Weight)) +
    geom_bar(stat="identity") +
    geom_text(aes(label=Weight), vjust=-0.2)

Figure 3-22. Left: labels under the tops of bars; right: labels above bars