[Rd] Lattice: Drawing a single contour line with a negative value fails

[Thorn Thaler]

Hi everybody,

I want to add a single contourline to a levelplot. While everything 
works fine if the value at which the line should be drawn is positive, 
there is an error if the value is negative:

library(lattice)
my.panel <- function(..., at, contour=FALSE, labels=NULL) {
    panel.levelplot(..., at=at, contour=contour, labels=labels)
    panel.contourplot(..., contour=TRUE, labels="Contour", lwd=2,
       at=con)
}

x <- y <- 1:100
df <- expand.grid(x=x, y=y)
df$z <- apply(df, 1, function(x)
    (x[1]^2+x[2]^2)*ifelse(x[1]^2+x[2]^2 < 5000, -1,1))
col.regions <- heat.colors(50)

# Works
con <- 1000
levelplot(df$z~df$x+df$y, col.regions=col.regions, panel=my.panel)

# Does not work
con <- -1000
levelplot(df$z~df$x+df$y, col.regions=col.regions, panel=my.panel)

I've tracked down the error to the function "cut.default", which takes 
an argument "breaks", which should be "either a numeric vector of two or 
more cut points or a single number (greater than or equal to 2) giving 
the number of intervals into which 'x' is to be cut."

So it seems to me that a single contourline at a positive value works 
because the value is interpreted as the number of intervals, even though 
there is just this single line in the resulting plot.

What would be the correct way to add just a single contour line to a 
levelplot, or is it indeed a bug?

Thanks,

Thorn