I can't check the manual now, but IIRC, the game is supposed to give a trade bonus along the longest uninterrupted trade route, which it will highlight if you cursor over your credit amount.
But what I'm seeing it show isn't nearly the longest route - it seems to cut bunches of planets out of the loop. In my current game, I have 12+ planets all with trade centers, and all connected, but at least 4-5 planets are left out that could have been put in the chain.
ie:
The letters are planets and the -'s and /'s are connections, ignore the .'s, which just keep the spacing correct:
....a-b
..././
w-x-y-z
The "longest" trade route I get shows w-x-y-z, with a and b ignored. But w-x-a-b-y-z does not revisit any planets and is a longer route both in terms of planet count and actual line distance traveled.
Am I missing something here? Is this a bug?
Also - is there any way to force my trade ships to stay within friendly space instead of taking shortcuts through enemy territory?