How to properly shade the background panels of an xyplot?

Dear R users,
this is a follow up of this message
http://tolstoy.newcastle.edu.au/R/e6/help/09/05/13897.html
I'm reproducing the core of it for convenience.

//
/ data(Oats, package = "MEMSS") /
/ tp1.oats <- xyplot(yield ~ nitro | Variety + Block, /
/ ? ? ? ? ? ? ? ? ? ? ?data = Oats, /
/ ? ? ? ? ? ? ? ? ? ? ?panel = function(x, y, subscripts, ...) { /
/ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?# How to normalize my heatmap metric with
/
/ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?# the value of the panel that has maximum
average ? /
/ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?# metric = eval(mean(y)/
max(<mean-of-each-panel>))))) /
/ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?metric = eval(mean(y)/max(Oats$yield)) /
/ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?panel.fill(col = gray(metric)) /
/ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?panel.lines(x,y) /
/ ? ? ? ? ? ? ? ? ? ? ? ? ?} /
/ ? ? ? ? ? ? ? ? ? ? ? ?) /
/ print(tp1.oats) /
//

xyplot(yield ~ nitro | Variety + Block,

? ? ? data = Oats,
? ? ? max.mean = max(with(Oats, tapply(yield, list(Variety, Block),
mean))),
? ? ? panel = function(x, y, subscripts, max.mean, ...) {
? ? ? ? ? metric = mean(y)/max.mean
? ? ? ? ? panel.fill(col = gray(metric))
? ? ? ? ? panel.lines(x,y)
? ? ? })


or

xyplot(yield ~ nitro | Variety + Block,

? ? ? data = Oats,
? ? ? aux.env = new.env(parent = emptyenv()),
? ? ? prepanel = function(x, y, aux.env, ...) {
? ? ? ? ? aux.env$max.mean.y <- max(aux.env$max.mean.y, mean(y))
? ? ? ? ? list()
? ? ? },
? ? ? panel = function(x, y, subscripts, aux.env, ...) {
? ? ? ? ? metric = mean(y) / aux.env$max.mean.y
? ? ? ? ? panel.fill(col = gray(metric))
? ? ? ? ? panel.lines(x,y)
? ? ? })

-Deepayan

The result is a trellis object in which the background colour of the panels
is an outcome of the data contained in the panel itself. After all, this is
what "panel = function (x,y ....." is meant for, right ?

But what, if I want to highlight some panels ? Arbitrarily or conditioned by
another variable.

Say I want to shade in gray only the upper right panels (Block VI, Victory
and Marvellous varieties )

Given a data frame like this, with a variable intended to set the colour of
the panel background
/
data(Oats, package = "MEMSS")
/Oats1 <- cbind.data.frame(Oats,
? ? ? ? ? ? ? ? ? ? ? ? ? ?Highlight =
? ? ? ? ? ? ? ? ? ? ? ? ? ?ifelse(Oats$Block == "VI" &
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Oats$Variety %in% c("Victory",
"Marvellous" ),
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? "gray",
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? "transparent")
? ? ? ? ? ? ? ? ? ? ? ? ? ?)

which is a possible code ?

I (more or less) know how to manage the data in the panel,
but I cannot imagine how to do it with variables external to the panel
itself.

I suppose that the panel functions are not useful here.
I'm wandering through par.settings, themes and panel.fill, but still without
success.
Any hint ?

--
Ottorino-Luca Pantani, Universit? di Firenze
Dip. Scienza del Suolo e Nutrizione della Pianta
P.zle Cascine 28 50144 Firenze Italia
Ubuntu 8.04.3 LTS -- GNU Emacs 23.0.60.1 (x86_64-pc-linux-gnu, GTK+ Version
2.12.9)
ESS version 5.5 -- R 2.9.2