That white line represents your longest chain of tradeports. The longer that chain, the bigger the multiplier on all the tradeport income in your empire.

Except the white line doesn't seem to be working correctly. In the image below I have two trade ports that should be considered part of the chain, but for some reason aren't.



Even odder, if I build a trade port at the southern most planet (the ice one), then the white line extends properly in the south, but is removed from one of the planets in the north.

Yeah, the longest route thing is totally bugged. Nothing you can do.

What he said. It finds the two farthest parts and then tries to draw the shortest 'chain' to connect them. Your two 'empty' ones are seperated by Space Junk and a Star, meaning the trade route goes through them, so they're effectively just neighbors. Therefore to draw the shortest, longest route, it must ignore those.

If you build one on the asteroid in the middle, it SHOULD route through there, and skip over the asteroid and ice planet, as it connects a shorter route.

In the first image I'd expect the trade route to make a crescent shape which includes all available trade ports on that chain. I don't see why an extra jump should be an issue.

I do agree that choosing the longest trade route is bugged. I played Dopplegangers earlier today and instead of running a trade route from my home planet, to its star, to the star of a conquered system, and to the star of a conquered enemy planet, it simply ran the trade route from a volcanic planet sitting next to the star in both systems. I've also seen a couple other scenarios pop up like when the route switches from a longer chain to a smaller chain because it might wrap around itself. It's very strange.