Here is a thread that might help. 

http://groups.google.com/group/ggplot2/browse_thread/thread/7f4238b5658e85bb/25d4cfb6215b7544?lnk=gst&q=circle+size#25d4cfb6215b7544
On Tuesday, May 10, 2011 at 12:15 PM, Alaios wrote: 
> Here you are :)
> 
> 
> 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))) 
> 
> }
> 
> 
> f<-matrix(data=seq(1:10000),nrow=100,ncol=100)
> plot_shad_f(f)
> 
> 
> I would like to add a circle at the middle of this region with a range of 10.
> 
> Best Regards
> 
> Alex
> 
> --- On Tue, 5/10/11, Scott Chamberlain <scttchamberlain4@gmail.com> wrote:
> > 
> > From: Scott Chamberlain <scttchamberlain4@gmail.com>
> > Subject: Re: [R] ggplot2 and add circle
> > To: "Alaios" <alaios@yahoo.com>
> > Cc: R-help@r-project.org
> > Date: Tuesday, May 10, 2011, 5:59 PM
> > 
> > You should provide reproducible data in addition to your code. 
> > 
> > S
> > On Tuesday, May 10, 2011 at 11:54 AM, Alaios wrote:
> > > 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
> > > 
> > > ______________________________________________
> > > R-help@r-project.org mailing list
> > > https://stat.ethz.ch/mailman/listinfo/r-help
> > > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> > > and provide commented, minimal, self-contained, reproducible code.
> > > 
> > 

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