plot multiple data sets on same axis

I've tried replacing the for loop body with this:

    this_method <- split.df[[i]]
    boxplot(elapsed~inactive, data=this_method,
            add=TRUE, border=i, boxfill=i, outline=FALSE)

but it has two problems:

* it doesn't plot at the correct x values. It looks like I need to
supply a list of the x values as the "at" parameter, and I don't know
how to get unique values from this_method$inactive. (I tried the clunky
labels(split(this_method, this_method$inactive)), but it returns them as
strings.)

* it redraws the graph's frame, and it ignores bty="l" when doing so.