The relevant code (in facet-grid-.r) seems to be:

margin_vars <- list(intersect(names(rows), names(data)),intersect(names(cols), names(data)))
data <- reshape2::add_margins(data, margin_vars, params$margins)
facet_vals <- quoted_df(data, c(rows, cols), params$plot_env)

The call to quoted_df() handles the expression evaluation needed to add the values that are missing, but this comes after the call to add_margins().

The possible solution I found is to replace those lines with:

data <- quoted_df(data, as.quoted(unique(c(names(data), names(rows), names(cols)))), params$plot_env)
margin_vars <- list(intersect(names(rows), names(data)),intersect(names(cols), names(data)))
data <- reshape2::add_margins(data, margin_vars, params$margins)
sym <- do.call(c, lapply(c(rows, cols), function(x) as.quoted(as.symbol(as.character(as.expression(x))))))
facet_vals <- quoted_df(data, sym, params$plot_env)

Since I don't have much facility with conversion from calls to expressions to quoted symbols, I'm sure there is a prettier way. But the idea is to first add in to the data frame data columns representing the faceting variables when they are expressions (thus the first call to quoted_df()).

@hadley Let me know if this seems like the right direction.

Thanks

@DanRuderman, you need to pull from master.

@has2k1 You're right. But why does % git status give output that makes it look like I'm up to date?
On branch master
Your branch is up-to-date with 'hadley/master'.

The relevant output from % git remote -v is:
hadley https://github.com/hadley/ggplot2.git (fetch)
hadley https://github.com/hadley/ggplot2.git (push)

git status is only as accurate as the state created by last git fetch. git status on its own does not initiate a network connection and so it does not show "real time" information.

After playing with this for a while, I'm not sure this can be done without completely rewriting how facets map data. For a few reasons, this may be a good thing (#2689 comes to mind).

Would it be possible to detect this and issue a warning? Or at least document it in the help page?