[R] ggplot2 and add circle

[Alaios]

Dear all,
today I have writted the following code,
to plot the contents of some matrices I have

plot_shad_f
function(f){
  library(ggplot2)
  dev.new()
  plotdata<-melt(f)
  names(plotdata)<-c('x','y','z')
  v<-ggplot(plotdata, aes(x, y, z = z))
  print(v + geom_tile(aes(fill=z))) 
}

I would like to ask your help add a small circle in this plotting. What would be the easiest way to do that in ggplot2?

Best Regards
Alex