[R] multiple xyplots on the same graph

[S. Swaminathan]

I am new to R and am having trouble plotting multiple xyplots on the same 
graph.
I have a dataframe x1 with 3 columns month, var1, var2. Month is a factor 
while var1 and var2 are numeric.
Reading through the archives I learned to plot var1 vs month and var2 vs 
month and have them as two plots in one window by doing the following
library(lattice)
x<-xyplot(var1~month,type="b")
y<-xyplot(var2~month,type="b")
print(x,split(c(1,1,1,2),more=TRUE)
print(y,split(c(1,2,1,2))

How do I get just one plot with month as the x axis and two lines for var1 
and var2 ?

thanks
swami