continuous plot over barplot

["David Parkhurst" <parkhurs@ariel.ucs.indiana.edu>]

I'm lecturing on the normal approximation to the binomial, and want to overlay the two onto a single plot.  I tried this:

 

x <- 60:100
y <- dbinom(x,100,0.7)
xy <-d ata.frame(x,y)
plot(x,y)
yy <- dnorm(x,70,sqrt(21))
lines(x,yy)

but at the lines command, a second (but blank) graphics window opens.  How can I overlay the normal plot on the barplot of the binomial?

 

Thanks.

 

Dave Parkhurst