[patch] add=TRUE in plot.default()

Hi Duncan,

It has never been claimed that it would work, and as far as I can see, 
it doesn't make anything easier:  the last line could be replaced by

lines(X, f(X), col="red")

for more clarity from less typing.  So why would you want add=TRUE in 
plot.default?

I don't like lines() because it ONLY allows you to add to plots.  I could
send a patch for that too...

But I also like how plot() is polymorphic.  It's nice how you can do some
computation -- a regression, a histogram or whatever, and then just call plot()
on it, and you have a nice graphical representation of it.  But why stop
there... wouldn't it be nice if you could stack them on top of each other?
It's often useful to compare things by putting them on top of each other.
(Are 2 regression lines the same or similar?  Are two utility functions the
same? etc.)

Cheers,
Andrew