R help - Jan 2007 - zero margin / marginless plots

Hi,

I'd like to produce a marginless or zero margin plot so that the pixel 
coordinates represent the mathematics.

xy<-data.frame(x=c(0,1,1,0,0),y=c(0,1,0,0,1))
png('junk.png',width=300,height=300)
par(mar=c(0,0,0,0))
plot(xy$x,xy$y,xlim=c(0,1),ylim=c(,1))
dev.off()

The resultant file has about a 10 pixel margin around these lines, and I'm 
not sure what parameter or function is controlling this offset.  Any 
hints?

Thanks for your time,
Dave
-- 
  Dr. David Forrest
  drf at vims.edu                                    (804)684-7900w
  drf5n at maplepark.com                             (804)642-0662h
                                    http://maplepark.com/~drf5n/

On Wed, 2007-01-10 at 21:18 -0600, David Forrest wrote:
> Hi,
> 
> I'd like to produce a marginless or zero margin plot so that the pixel 
> coordinates represent the mathematics.
> 
> xy<-data.frame(x=c(0,1,1,0,0),y=c(0,1,0,0,1))
> png('junk.png',width=300,height=300)
> par(mar=c(0,0,0,0))
> plot(xy$x,xy$y,xlim=c(0,1),ylim=c(,1))
> dev.off()
> 
> The resultant file has about a 10 pixel margin around these lines, and
I'm
> not sure what parameter or function is controlling this offset.  Any 
> hints?
> 
> Thanks for your time,
> Dave
By default, the axis ranges are extended by +/- 4%.  You can change this
by using:

  plot(xy$x, xy$y, xlim = c(0, 1), ylim = c(0, 1), 
       xaxs = "i", yaxs = "i")

where 'xaxs' and 'yaxs' set the axis ranges to the actual data
ranges.

See ?par for more information.

HTH,

Marc Schwartz