Plugin graphics not removable after saved session.

[mlangill]

If a plugin (e.g. Linear Regression or RCA) creates visualizations such as bar plots on the GenGIS map, and the user saves a session before closing the plugin window, then loads from that session later there will be bar plots that can not be removed (opening and closing the plugin window does not help).

In fact even removing and re-adding the map will still show the graphics, which means the session is ruined and a new session must be used.

[rbeiko]

Tentatively putting this in 2.2. Maybe Morgan can give Timothy a short demonstration of the problem?