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

Part of the reason I didn't like your patch is that it was incomplete: 
it didn't patch the plot.default.Rd file.

Fair enough -- I wasn't sure whether I was fixing a bug or not.  ("..."
spreads the documentation around a bit.)

That function already has 
around 16 parameters; do we really want to add another one, that 
interacts with some of the ones that are there?

Yes.  The ability to plot things on top of each other is important.
The simplicity created by having a single interface for adding to plots
outweighs the complexity of yet another parameter.

The add parameter only interacts with other parameters superficially --
some parameters of "plot" (like log) are related to the shape of the axes,
and should be inherited from what is on the plot already.

What you really seem to want is to add it to the generic plot(),

Agreed.

but it's way too late to go modifying that particular generic.

I agree.  Adding an "add=FALSE" parameter to plot() would generate errors for
methods that don't implement it, so they would all have to be changed
simultaneously, including in private/unreleased code.

So I'd like to settle for second best: adding add=FALSE parameters to many plot
methods.

Cheers,
Andrew