Adding legends on plots using Lattice package

To dear Dr Sarkar and anyone that knows about Lattice package,

   I make 4 graphs by Lattice package. Each of the graphs has two time
series. All the series are plotted in plain lines by default, and I would
like one series to be in plain line and the other to be in dotted line in
each graph. How can I modify the command of xyplot in the following line to
achieve this? It seems that "key" or "auto.key"  parameters are needed, but
I don't know how to do it. Thank you very much!


require(graphics)
library(lattice)
data1<-read.csv(file="G_Results_3FX1.csv", header=TRUE)

xts<-ts(data1[,2:9],frequency = 4, start = c(1983, 1))
xyplot(xts, screen=list("a","a","b","b","c","c","d","d"), layout=c(2,2),
scales=list(x="same",y="same"))



Miao