John, When panning the viewport through a plot shouldn't

    > the panning stop and ignore your request when you reach the
    > start or end of the plot. Is there a benefit to being able
    > to pan off the edge of a plot?

I like to see the data scroll past the end so I know I am truly at the
end. If nothing happens, I might think the mouse click didn't work,
the program froze, or something like that. Putting the question back
on you, is there a significant cost to doing it the way it is? Most
users don't continue panning where there is no data, I assume. It's
kind of in line with the python philosophy: we're all adults here; if
you want to pan through empty space that's your decision.