From the documentation for ggplot2's geom_tile() function, we have the following simple plot:

> # Generate data 
> pp <- function (n,r=4) { 
+  x <- seq(-r*pi, r*pi, len=n) 
+  df <- expand.grid(x=x, y=x) 
+  df$r <- sqrt(df$x^2 + df$y^2) 
+  df$z <- cos(df$r^2)*exp(-df$r/6) 
+  df 
+ } 
> p <- ggplot(pp(20), aes(x=x,y=y)) 
>  
> p + geom_tile()

How do I remove the margins that border the tile?