I now have the app pulling back a drop-down list of cities, based on the years selected, and then generating lists of tracts for each year. There is the problem, though, that some cities don't have data for all the years you submit. I'm waiting for feedback from JS-R about whether I should filter the city selection list to only those cities for which there's data for ALL the years you've selected. I think that should be the case, but I'm not quite sure.