[R] xlim=c(min(x), max(x)) still produces extra margin on both sides of x axis in plot()

[madr]

code:
op <-
par(bg='black',fg='gray',col='gray',col.axis='gray',col.lab='gray',col.main='gray',col.sub='gray',mai=c(0,0,0,0),
tck = 0.01, mgp = c(0, -1.4, 0), mar=c(0,0,0,0))
plot(x,y,ylim=c(-20,20),xlim=c(min(x),max(x)),pch='X',col = rgb(1,1,1,
0.5),yaxt="n", ann=FALSE)
abline(v=c(min(x),max(x)), lty=3, col="yellow")
par <- op

render:
http://i51.tinypic.com/2rz9w0h.png

So what I mean is the area between yellow lines and edges of the plotting
area, so min(x) and max(x) would be really on the edge of the plot.
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