This will allow us to add more customization to the class without
having a static function with an exceedingly long list of arguments...

Signed-off-by: Patrick Avery <psavery@buffalo.edu>

Now, the old VtkPlot objects are re-used instead
of being deleted and re-created.

This prevents an issue where deleting the old object
would sometimes result in crashing due to the Qt
event loop.

Signed-off-by: Patrick Avery <psavery@buffalo.edu>