You want to add line segments or arrows to a plot.

Use annotate("segment"). In this example, we’ll use the climate data set and use a subset of data from the Berkeley source (Figure 7-10):

library(gcookbook) # For the data set

p <- ggplot(subset(climate, Source=="Berkeley"), aes(x=Year, y=Anomaly10y)) +
     geom_line()

p + annotate("segment", x=1950, xend=1980, y=-.25, yend=-.25)

Figure 7-10. Line segment annotation

It’s possible to add arrowheads or flat ends to the line segments, using arrow() from the grid package. In this example, we’ll do both (Figure 7-11):

library(grid)
p + annotate("segment", x=1850, xend=1820, y=-.8, yend=-.95, colour="blue",
             size=2, arrow=arrow()) +
    annotate("segment", x=1950, xend=1980, y=-.25, yend=-.25,
             arrow=arrow(ends="both", angle=90, length=unit(.2,"cm")))