hadley/ggplot2 — pull request #598 by kohske (2012-06-26 01:17:06+00:00)
Title: arrow of geom-path can be filled.

https://groups.google.com/forum/?fromgroups#!topic/ggplot2/QkdrXgj8tWk

```{r}
ggplot(mtcars, aes(x=wt, y=mpg, colour=cyl))+
geom_path(arrow=arrow(type="closed"))
ggplot(mtcars, aes(x=wt, y=mpg, colour=cyl, fill = cyl))+
geom_path(arrow=arrow(type="closed"))
ggplot(mtcars, aes(x=wt, y=mpg, colour=cyl, fill = cyl))+
geom_path(arrow=arrow(type="open"))
```

I set white as default, in compatible with the previous behavior.
But maybe NA default, i.e., transparent, is better.