Still Breathing

So I'm starting to plan this sci-fi tabletop game which features stable wormholes linking locations (think of a less-cheesy version of Stargate: Atlantis or an inferior version of The Algebraist) and I'm wondering if there's some kind of (easy to use and free) mapping programme that anyone knows of to show solar systems and their relationship to each other based on the wormholes?

Any help gratefully received.