Question

In
Adding individual arrows in multiple plots
I described a problem which has been resolved thanks to mathematical.coffee.

My problem was how to add a single arrow to only one facet. Recently I installed a new version of R:

platform i386-pc-mingw32
version.string R version 2.14.2 (2012-02-29) ggplot2_0.9.0

Here is the code example

# data frame
xdf <- data.frame(x=rep(1:10,2)
                  ,y=c( 2*c(1:10)+rnorm(10,0,3), 4*c(1:10)+rnorm(10,0,5))
                  ,z=rep(c("A","B"),each=10)
                  )
xdf

# plot
ggplot with faceting
xp <- ggplot(xdf,aes(x=x,y=y)) +
   geom_line() +
   facet_grid(. ~ z)
xp

# location of the arrow: x=4, y=y+1 on the top of the first facet (A)
(f1x4 <- xdf[4,"y"]+1)

# add arrow and label
xp + geom_segment(aes(x=4,xend=4,y=f1x4+3,yend=f1x4,z='A')  # <-- see the z='A'
                  , arrow=arrow(length=unit(0.4,"cm")
                    )
                 ) +
  geom_text(aes(x=4,y=f1x4+5, label="a",z='A'))        # <-- see the z='A'

What should happen: the arrow should be created only on facet A.
What happens: the arrow is created on both facets A and B.

Has anybody an idea how to solve this problem?

Answer 1

Obviously there was a change after updating R and ggplot. The coordinate data have to be delivered with a data frame. Here is an example with an additional grouping:

xdf <- data.frame(x=rep(1:10,each=4)
              ,y=rep(1:10,each=4)*rep(1:4,10)  +rnorm(40,0,1)
              ,g=rep(c("R","S"),20)
              ,z=rep(c("A","A","B","B"),10)
              )
head(xdf)
# plot
xp <- ggplot(xdf,aes(x=x,y=y, group=g)) +
geom_line() +
facet_grid(. ~ z)
xp
# location of the arrow: x=4, y=y+1 on the top of the first facet (A)
(f1x4 <- subset(xdf,x==4 & g=="R" & z=="A")$y)
arrdf <- data.frame(x = 4, y = f1x4, z = "A", g = "R")   # create new data.frame for    annotations
# add arrow and label
xp + geom_segment(data=arrdf,aes(x=x,xend=x,y=y+3,yend=y,z=z,g=g)  # <-- see the z='A'
              , arrow=arrow(length=unit(0.4,"cm")
                )
             ) +
geom_text(data=arrdf,aes(x=x,y=y+4, label="a",z=z, g=g))