On 8/19/09, Thorn Thaler <thot...@sbox.tugraz.at> wrote:
> 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?
Definitely a bug. It should be easy to fix (but I haven't had time to
get to it yet).
Using something like
con <- c(-1000, NA)
seems to work though.
-Deepayan
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