Segment annotation on log10 scale works differently for the end and the beginning of the segment?

Question

I found a rather confusing feature in ggplot while trying to annotate segments on log10 scale. Following code produces the plot below:

library(ggplot2)
dat <- data.frame(x = x <- 1:1000, y = log(x)) 
ggplot(dat, aes(x = x, y = y)) +
geom_line(size = 2) + scale_x_log10() +
annotate("segment", x = 0, xend = log10(100), y = log(100), yend = log(100), linetype = 2) +
annotate("segment", x = log10(100), xend = log10(100), y = 0, yend = log(100), linetype = 2)

Whereas this is what I am after:

ggplot(dat, aes(x = x, y = y)) +
geom_line(size = 2) + scale_x_log10() +
annotate("segment", x = 0, xend = log10(100), y = log(100), yend = log(100), linetype = 2) +
annotate("segment", x = 100, xend = log10(100), y = 0, yend = log(100), linetype = 2)

In other words, I have to log10 transform the endpoint of the segment on x-axis, but not the beginning. Does this behaviour have a logical explanation? I understand that aes() does the transformations...but in this case, transformations on x-axis should be uniform (well, log10), right?

I am working on:

R version 3.0.0 (2013-04-03)
Platform: x86_64-w64-mingw32/x64 (64-bit)
ggplot2_0.9.3.1 

Answer 1

Found that this is a bug of scales() (not only for the scale_x_log10()) when it is used with annotate() and xend value is provided (it is already filled as issue by W.Chang). In this case transformation of xend is done only in one direction - log10 of value is not taked but power is calculated.

scale_x_log10() works without problems if, for example, "rect" is used in annotate() and xmin, xmax values are provided.

ggplot(dat,aes(x,y))+geom_line()+
  scale_x_log10()+
  annotate("rect",xmin=100,xmax=1000,ymin=log(10),ymax=log(200))

Workaround for this problem would be to use geom_segment() with data=NULL and all other values put inside the aes().

ggplot(dat, aes(x = x, y = y)) +
  geom_line(size = 2) + scale_x_log10() +
  geom_segment(data=NULL,aes(x = 100, xend = 100, y = 0, yend = log(100)), 
                                                             linetype = 2)