[R] how to get rid of unused space on all 4 borders in plot() render

[madr]

x= c(1,5,7,-3,4)
y= c(2,4,-5,2,5)
plot(x,y,ylim=c(-20,20),xlim=c(min(x),max(x)),pch='X',col = rgb(0, 0, 0,
0.5),yaxt="n", ann=FALSE)

and this code produces:
http://i53.tinypic.com/ffd7d3.png

Where I marked in red areas that I want to get rid of and use as much real
screen estate as I can.
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