Traveling from Chartes….on the way to Barcelona eventually

That's a tough one. Chartres is practically in the Paris region, on a local train line, so going back through Paris is the logical choice. Going south from Chartres, you are going down the west coast of France. Barcelona is on the east coast of Spain/Catalonia, separated by mountains. Sorry, I can't give you more information without studying lesser known train and bus connections.

Thanks…I thought as much, it was worth a try!!

Since you'll need to go back to Paris Orly airport to travel to Barcelona, I would just extend my stay somewhere close to Barcelona, like Girona. There is nowhere near Chartres that would make sense for just 1-2 nights.