Setting the background colour of a polygon
===

Is there a way to set the background colour of a polygon? I want to get a plot like this...

    set.seed(1)
    n <- 100
    xx <- c(0:n, n:0)
    yy <- c(c(0,cumsum(stats::rnorm(n))), rev(c(0,cumsum(stats::rnorm(n)))))

    plot   (xx, yy, type="n", xlab="Time", ylab="Distance")
    polygon(xx, yy, angle=45, density=10)
    polygon(xx, yy+5, col="white")
    polygon(xx, yy+5, angle=45, density=10, col="red")

But ideally without the penultimate line to set the background (I am plotting multiple polygons within a function I am writing). Is there an argument I can use in the final line that will negate the whole of the penultimate line? Cheers.

![Ugliness][1]


  [1]: http://i.stack.imgur.com/VNgyP.png