[R] How to properly shade the background panels of an xyplot?

[Ottorino-Luca Pantani]

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, Università 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