[R] tick.number for date in xyplot

[Benjamin Tyner]

I would like a tick mark for each month; for example,

xyplot(runif(365)~I(as.Date("1999-01-01") + 1:365),
       scales=list(x=list(format="%b %Y",tick.number=12)))

I know I could make x numeric and use 'at' and 'labels', but I was
wondering if there is a more direct route I'm missing. (In particular,
one that doesn't have to be modified for new data).

Thanks,
Ben