library(lattice)
barchart.gray <- function(data.long, v.spc, relative = FALSE)
{
	n.spc <- length(unique(data.long$spc))
	v.forest <- sort(unique(as.character(data.long$forest)))

	superpose.polygon <- trellis.par.get("superpose.polygon")
	superpose.polygon$col <- gray(1:n.spc / n.spc)
	trellis.par.set("superpose.polygon", superpose.polygon)

	func.strip <- function(which.panel, ...)
	{
		lrect(0, 0, 1, 1, col = "#eeeeee")
		ltext(0.5, 0.5, v.forest[which.panel])
	}
	print(
		barchart(
			n ~ treatment | forest,
			groups = spc,
			data = data.long,
			stack = TRUE,
			auto.key = list(
				points = FALSE,
				lines = FALSE,
				rectangles = TRUE,
				space = "right",
				reverse.rows = TRUE
			),
			ylab = "",
			layout = c(length(v.forest), 1),
			strip = func.strip
		)
	)
}