filter legend when you filter layer

A really useful feature enhancement would be the ability to have the legend display filtered when you filter the layer.  so if a user filters a layer by a a;ttribute selection or by an area, the legend would be filtered accordingly.  if you filtered by map extent, then you could have a dynamic legend that  relates to only the features in the view and in the selection.  After looking at json help for arcserver, this seems do-able,  I think this would be a valuable feature in essentials.

jeff