The automatic chart overlay feature is good when it works, but it seems to not work I'm situations when more than one chart is available.

It's kind of frustrating that this is the case, because it's usually exceedingly obvious which one to display. I can think of some heuristics that could be used. But also, if that's deemed impractical or inelegant or whatever by the developers, then how about giving users some control if this? I should be able to state when planning a route which chart I would like overlayed depending on my position in the nav plan.

So you'd want to somehow nominate one (and only one, presumably) document as being the one you'd want to open automatically when you came close to its map boundaries?