R devel - Aug 2009 - Lattice: Drawing a single contour line with a negative value fails

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

On 8/19/09, Thorn Thaler <thothal at 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