Plugins are re-initialized with each re-plot (which may not be the right
thing to do, but that's how it works for now).  The previous approach of
saving references to the original Canvas functions therefore broke,
since the second time around we'd get a reference to our new function.
Instead we hold those references as globals within the plugin, and only
set them once.

This whole idea of replacing prototype functions is, now that I step
back and look at it, really awful.  This needs to be changed ASAP to
something less ridiculous.