I would think exporting to a detailed csv would be the best bet for what you are trying to do. VS1 separates gps/time information from AP information. The detailed CSV format is more like a log format and would be easier to match specific times with a access point.
As for the graphing i would say thats one of vistumblers weak points. The graphing feature is very basic GDI+ and I have never figured out how to indicate times horizatally. Think of it more like a "at this moment" signal visualization. One tip I have is by default "View -> Graph Dead Time" is unchecked. This makes the graph so it only shows one line of dead time. If you turn on graphing dead time it will show a line for every second was inactive.