Feature request: preserve selected plots when changing tract/visit/etc.

[rmjarvis]

Currently, if I want to look at some particular plot(s) over several different tracts (or visits presumably, but I couldn't quickly find a collection that had any visit-based plots), then I need to reselect the set of plots after changing the tract number. E.g. if I select the 6 rho stats plots, then change the tract, the plots are deselected and I need to select them again.

Suggestion: if the new tract has the same plots available, then automatically select the corresponding plots for that tract when changing the tract number.