hadley/ggplot2 — pull request #654 by wch (2012-08-26 05:17:30+00:00)
Title: Add facet headers to facet_grid

This is a first pass at adding facet headers. Examples here: http://rpubs.com/wch/1306

Things that need to be done:

* Improve appearance
* Decide whether to allow this for `facet_wrap`, and if so, how
* Control over whether each header is drawn
* Syntax for setting header labels to something other than the default
* Figure out what to do when facet has two variables on one axis, like `facet_grid(. ~ am + cyl)`

Maybe the labels could be controlled like this:

```
facet_grid(am ~ cyl, titles = "auto/manual" ~ NULL)
```

Where `NULL` means not to draw facet headers for that variable.