[R] lattice/levelplot: panels with values can be empty

[Wolfram Fischer]

I tried:
	library(lattice)

	F0  <- c( 'A', 'A', 'B', 'B' )
	F1  <- c(  1 ,  1 ,  1 ,  2 )
	F2  <- c(  8 ,  9 ,  8 ,  9 )
	VAL <- c(  20,  50,  10,  60 )
	df <- data.frame( F0, F1, F2, VAL )

	levelplot( VAL ~ F1 * F2 | F0, data=df )

I got an empty field for F0 == 'A'
and a colored field for F0 == 'B'.
I expected two colored fields. - What can I do?

Thanks. Wolfram