[Rd] [R] NA in first panel of xyplot causes error (PR#2349)

[wolfram@fischer-zim.ch]

Problem:
If the selection of values for the first panel of an xyplot
leads to a vector of NAs, an error is produced:
	Error in pretty(x[is.finite(x)], ...) : x must be numeric

Example:
	xa <- 1:8
	xb <- rep( c( NA, 10 ), 4 )
	xc <- rep( c( 'C1', 'C2' ), 4 )
	xyplot( xa ~ xb | xc )

Remark:
Changing the data for the first and the second panel eliminates
the error message:
	xb <- rep( c( 10, NA ), 4 )
	xyplot( xa ~ xb | xc )

Version of R:
	R 1.6.1 on Linux

Wolfram