Solid and dotted lines saving an xyplot

Dear R-users,
I would a small problem saving a graph with postscript after using
xyplot.
Whereas in the R-screen the two lines are solid, once the image is saved
as .eps file the second lines (blue in the following example) is dotted.
Here there is a simple example.
Thanks in advance for your help,
Carlo Giovanni Camarda

# creating data
a1 <- sort(runif(10))
a2 <- sort(runif(10))
a3 <- sort(runif(10))
a4 <- sort(runif(10))
a <- c(a1,a2,a3,a4)
b1 <- sort(runif(10))
b2 <- sort(runif(10))
b3 <- sort(runif(10))
b4 <- sort(runif(10))
b <- c(b1,b2,b3,b4)
c <- rep(1:10,4)
d <- rep(1:4, each=10)
# creating a dataset
df <- data.frame("a"=a,"b"=b,"c"=c,"d"=d)
# plotting in R (look the solid lines)
xyplot(a + b ~ c | d, data = df, col=c("red", "blue"), type="l",
layout=c(2,2))
# saving somewhere (one solid and one dotted line)
postscript("example.eps")
xyplot(a + b ~ c | d, data = df, col=c("red", "blue"), type="l",
layout=c(2,2))
dev.off()



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