Local Variables Within aes

I'm trying to use a local variable in aes when I plot with ggplot. This is my problem boiled down to the essence:

xy <- data.frame(x=1:10,y=1:10)

plotfunc <- function(Data,YMul=2){
    ggplot(Data,aes(x=x,y=y*YMul))+geom_line()
}

plotfunc(xy)

This results in the following error:

Error in eval(expr, envir, enclos) : object 'YMul' not found

It seems as if I cannot use local variables (or function arguments) in aes. Could it be that it occurrs due to the content of aes being executed later when the local variable is out of scope? How can I avoid this problem (other than not using the local variable within aes)?

I would capture the local environment,

xy <- data.frame(x=1:10,y=1:10)

plotfunc <- function(Data, YMul = 2){
    .e <- environment()
    ggplot(Data, aes(x = x, y = y*YMul), environment = .e) + geom_line()
}

plotfunc(xy)

Here's an alternative that allows you to pass in any value through the YMul argument without having to add it to the Data data.frame or to the global environment:

plotfunc <- function(Data, YMul = 2){
    eval(substitute(
        expr = {
            ggplot(Data,aes(x=x,y=y*YMul)) + geom_line()
        }, 
        env = list(YMul=YMul)))
    }

plotfunc(xy, YMul=100)

To see how this works, try out the following line in isolation:

substitute({ggplot(Data, aes(x=x, y=y*YMul))}, list(YMul=100))