Fix issues related to refactoring of view-level
to subplot-level behavior, such that previous
functionality is restored. This refactoring
supports introduction of a stacked-plot mode,
for WTD-625.
Begin refactoring Plot view to support stacked plots;
separate out behavior which needs to occur per-plot
from area which should occur per-view, and add a
handler for the Overlay mode. WTD-625.