[R] pairs, par("plt")

[Oliver Soong]

I'm having some confusion over the coordinate system after using
pairs.  I'm not interested in the content of the actual pairs plot,
although the number of pairs seems to matter a bit.  I'm purely
interested in knowing where my points will be plotted on the device.
However, after using pairs, the par information (omd, fig, plt, and
usr) don't reflect what points does.  For example:

pairs(iris[1:5])
par(xpd = NA)
points(0 - 0.01 * 1:100, 0 - 0.01 * 1:100)
points(0 - 0.01 * 1:100, 1 + 0.01 * 1:100)
points(1 + 0.01 * 1:100, 0 - 0.01 * 1:100)
points(1 + 0.01 * 1:100, 1 + 0.01 * 1:100)
par(c("omd", "fig", "plt", "usr"))

The resulting plot shows that the corners of the are approximately
0.05 user coordinate units from the boundaries of the plot region.
According to par, though, there is a margin around the plotting region
that is clearly not symmetric and does not correspond to around 0.05
units.

If we use pairs(iris[1:2]) and repeat the rest, the corners are now
0.02 user coordinate units.  par provides the same information as
before.

So:
1. How do I figure out where coordinates I give to points will display
on the figure?
2. More generally (for my own understanding), why does the par
information not do what I expect?  Do I have some fundamental
misunderstanding of the arrangement of plotting, figure, display, and
margin regions within the device?

I'm using R 2.5.1, and this behavior occurs on a fresh R console.

Thanks!

Oliver


-- 
Oliver Soong
Donald Bren School of Environmental Science & Management
University of California, Santa Barbara
Santa Barbara, CA 93106-5131
805-893-7044 (office)
610-291-9706 (cell)