[plt-scheme] lines aren't complete

It's an sgl kind of night :-)

I've noticed that on re-paints my lines are not getting drawn
properly.  Many bits are skipped, _especially_ on lines which are
horizontal or vertical.

I can't figure out if this is due to the way sgl is updating buffers -
since I don't understand how it is - or whether it's an incomplete
understanding of open-gl on my part.

I do _not_ think it has anything to do with anti-aliasing or lack thereof.

clues ?

Thanks


Brian