按照老师的程序输入为什么遇到了layout错误？

set.seed(1)

x <- rnorm(100) 

f <- rep(0:1, each = 50) 

y <- x + f - f*x + rnorm(100, sd = 0.5) 

f <- factor(f, labels = c("Group1", "Group2")) 

xyplot(y~x | f, layout(2,1)) 

然后为啥出现错误？ 

Error in layout(2, 1) : layout matrix must contain at least one reference to each of the values {1 ... 2}