Overlay histogram with density curve

I am trying to make a histogram of density values and overlay that with the curve of a density function (not the density estimate).

Using a simple standard normal example, here is some data:

x <- rnorm(1000)

I can do:

q <- qplot( x, geom="histogram")
q + stat_function( fun = dnorm )

but this gives the scale of the histogram in frequencies and not densities. with ..density.. I can get the proper scale on the histogram:

q <- qplot( x,..density.., geom="histogram")
q

But now this gives an error:

q + stat_function( fun = dnorm )

Is there something I am not seeing?

Another question, is there a way to plot the curve of a function, like curve(), but then not as layer?