Annotations - R Graphics - page 204

Adding Line Segments and Arrows
Problem
You want to add line segments or arrows to a plot.
Solution
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
Discussion
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" )))