missing factor levels in a lattice barchart panel cause unexpected failure

I think I've found the problem, and a sort of fix, for this issue.

It appears in the panel.barchart function

each of the clauses in the function has a set of lines roughly like:

             groups <- as.numeric(groupSub(groups, ...))
             vals <- sort(unique(groups))
             nvals <- length(vals)
             col <- rep(col, length = nvals)
             border <- rep(border, length = nvals)
             lty <- rep(lty, length = nvals)
             lwd <- rep(lwd, length = nvals)
             height <- box.ratio/(1 + nvals * box.ratio)
             if (reference)
                 panel.abline(v = origin, col = reference.line$col,
                   lty = reference.line$lty, lwd = reference.line$lwd)
             for (i in unique(y)) {
                 ok <- y == i
                 nok <- sum(ok, na.rm = TRUE)
                 panel.rect(x = rep(origin, nok), y = (i + height *
                   (groups[ok] - (nvals + 1)/2)), col = col[groups[ok]],
                   border = border[groups[ok]], lty = lty[groups[ok]],
                   lwd = lwd[groups[ok]], height = rep(height,
                     nok), width = x[ok] - origin, just = c("left",
                     "centre"))
             }

Which sets the parameter lty (and others) to NA in the example below.

D = data.frame(X=1, Y=factor(letters[2], letters[1:2]))
barchart(~ X, D, groups=Y)

This (NA) is because:

groups=[1] b
Levels: a b

thus the code then does

groups==2
vals==2
nvals==1
ok==TRUE

hence

groups[ok] == 2

but

length(lwd) == 1

thus

lwd[groups[ok]] == lwd[2] == NA

This is due to the mistaken assumption that the numeric component of
groups must be a subset of the 1:length(groups), when in fact it can
be a subset of 1:length(levels(groups)).


a silly fix:
----

replacing

             groups <- as.numeric(groupSub(groups, ...))
             vals <- sort(unique(groups))
             nvals <- length(vals)

with

             nvals <- length(levels(groups))
             groups <- as.numeric(groupSub(groups, ...))

fixes my example, but it clearly short of a full solution.