[R] levelplot behaviour for panel with constants

[Edzer J. Pebesma]

In the example:

x = rep(c(0,0,1,1),4)
y = rep(c(0,1,0,1),4)
z = c(1,0,1,0,0,0,1,1,0,1,0,0,1,1,1,1)
f = as.factor(c(rep("a",4),rep("b",4),rep("c",4),rep("d",4)))
levelplot(z~x+y|f,data.frame(x=x,y=y,z=z,f=f))

I noted that the last ("d") plot remains empty. I guess the
reason for this is that the values are constant (1), but I consider
it more consistent if they would get the colour of 1, and would
be left blank in case they were NA's.
--
Edzer