xyplot infelicity when a panel does not contain non-NA values

[tyner]

Reprex:

dat <- data.frame(x = 1:12,
                  p = gl(3L, 4L),
                  y = rep(c(NA, 1, 2), each = 4L)
                  )
                  
ylim <- list(c(0, 2), c(1, 3), c(2, 4))

p <- lattice::xyplot(y ~ x | p, 
                     data = dat, 
                     ylim = ylim,
                     scales = list(relation = "free")
                     )

The warning which comes from latice:::limitsFromLimitlist is somewhat cryptic:

In limitlist[id] <- lim :
  number of items to replace is not a multiple of replacement length

After some investigation it appears that when specifying per-panel ylim, it expects entries only for the subset of panels with non-NA values. However, as far as I can discern, this is not documented. If this is by design, it would be good to document, perhaps with some guidance around best practices (for example, is it better to remove NAs upfront, or is there a programmatic way to figure out the correct subset of ylim to pass, which might not be a trivial task if levels have been reordered and/or dropped (i.e. if the perm.cond and/or drop.unused.levels argument has been customized).

Thank you for considering.

[tyner]

Based on the comment in scales.R (line 348) it seems the user is expected to supply limits that conform to the output of the prepanel.

            ## There's a subtle consideration here.  It is possible
            ## for some panels to have nothing in them (or only NA's).
            ## Such panels usually have their prepanel functions
            ## return NA.  When 'limits' is specified as a list, this
            ## will be interpreted as the limit specification for the
            ## non-empty panels only (this is an arbitrary choice, but
            ## it usually makes more sense, even though it's less
            ## general than the other choice).