ggplot2 geom_area plots unexpectedly (ends in wrong place)

I'm using ggplot2 to plot the following data using `geom_area`. The values are designed to be a x axis mirror image. But the end of the plot for dog is not mirrored. But as you can see from this data frame dog should end on x=100 and y=0 but it seems to end on x=100 and y=100 (see last row) How can I make dog mirror cat exactly? ``` x y animal 1 100.0 100 cat 2 90.0 89 cat 3 84.0 85 cat 4 55.5 60 cat 5 28.3 37 cat 6 27.0 32 cat 7 18.0 25 cat 8 0.0 0 cat 9 0.0 100 dog 10 10.0 89 dog 11 16.0 85 dog 12 44.5 60 dog 13 71.7 37 dog 14 73.0 32 dog 15 82.0 25 dog 16 100.0 0 dog dat <- structure(list(x = c(100, 90, 84, 55.5, 28.3, 27, 18, 0, 0, 10, 16, 44.5, 71.7, 73, 82, 100), y = c(100, 89, 85, 60, 37, 32, 25, 0, 100, 89, 85, 60, 37, 32, 25, 0), animal = c("cat", "cat", "cat", "cat", "cat", "cat", "cat", "cat", "dog", "dog", "dog", "dog", "dog", "dog", "dog", "dog")), class = "data.frame", row.names = c(NA, -16L), .Names = c("x", "y", "animal")) library(ggplot2) ggplot(dat) + theme_bw() + geom_area(aes(x=x, y=y, group=animal, fill=animal), alpha=.3) ``` Oddly, if I subset just dog it plots correctly: ``` ggplot(subset(dat, animal=="dog")) + theme_bw() + geom_area(aes(x=x, y=y, group=animal, fill=animal), alpha=.3) ```