I think you'll need to start by defining the FTL travel you use in this campaign. You say that the lines are trade routes, so that means that they do not necessarily indicate unidirectional FTL travel routes. So if you use omnidirectional FTL then these probably aren't very good trade routes because trade routes in space where there is no impedance to travel other than sheer time and distance trade routes will end up being much more like the "triangle route" established in the Atlantic in the 17th to 19th centuries (http://en.wikipedia.org/wiki/Triangular_trade) where goods that are cheap in one place are traded to a place where they are more costly but that location in turn has a trade good that is valuable somewhere else which in turn has a trade good that is valuable in the original location. Though it does not necessarily need to be only three ports, it could be two or far more than three. So in that case you'd want to indicate what goods are flowing in what directions. Also for example if FTL is omnidirectional ports like Tassah and Regea might be directly connected for some routes.
However if FTL is unidirectional (or more accurately N-directional) where each star system is connected via FTL only to a specific set of other stars (like Wing Commander or Starfire) then these trade routes might be better, but you probably need more connections and should still indicate the circular/triangular routes and what goods flow in what direction.
Or at least that's how I'd do it.
As for what you might need in the legend:
Scale if Omnidirectional or "Jump Distance" if N-Directional.
Trade route goods.