Someone can correct me if I'm wrong but I think in order to have a chain of trade routes they need to be linked in an unbroken chain with no gravity wells inbetween. The trade ports will need to connect. I'm currently in a game where I needed to fill some holes in the trade line I thought I had established but they were through one worm hole and one asteroid field. In order to link them I placed a starbase with trade facilities in them which linked the chain.

Ah! Thanks for clarifying the difference in trade between Sin 1.15 and Entrenchment 1.01

I noticed this as well. Does the order in which you build the trade ports (or in which you colonized the planets) have any influence? The white trade line shown usually isn't the longest possible in my games, not sure if it is just the graphics or trade income as well.

Trade chains have been changed in Entrenchment, they no longer automatically go thru uncolonizeable gravity wells. 

But they still only seek the "shortest longest" chain!  So if you have 8 tradeports connecting in a circle, but central connecting phase lanes, the trade chain will only be 2 long.

it has always annoyed me when an extra planet gets a trade port in the middle - but out of the way - of a trade rout and is not included, but in a recent game my trade routs litterally stop for no reason. I took this screen shot, hopefully it helps. every planet i own has a trade port on it, and every non-planet in my (red) culture also has a starbase with the trade port upgrade, but the trade port just stops

here is the independent link just in case: http://farm4.static.flickr.com/3609/3367116800_dd683d1970.jpg?v=0

and the picture

to get a longer chain you need to specifically not built ports on certain worlds that would make your optimal trade route shorter.

in the picture you presented you would have to build this trade port chain to get a longer route:

starting point: desert world on far west(left)

2nd: volcanic world to the east of the desert

3rd: terran world south-east of volcanic

4th: asteroid south-east of terran

5th: terran north of asteroid

6th: asteroid north-east of terran

that would give you a route with 5 lanes in it, which is 1 longer than your current route of 5 planets/4 lanes. you could go longer if you used star bases with trade docks, such as the route annatar suggests that uses the star and pirate base and an asteroid belt. my route uses only planets with natural logistics slots.

the value of the route is of course related to the number of lanes in the route. you ahve to be careful about which adjacent planets get ports or else you might break the lane. its worth noting that you can build as many ports as you want at worlds that are not adjacent to planets on the route and it won't break the chain at all. so in the example i described for your map, the ice world is totally non-adjacent to the route and you could fill it up with trade ports without affecting the length of your route at all.

if you find any of this stuff interesting in general, as opposed to just in terms of playing better in Sins, you might want to check out this:

http://en.wikipedia.org/wiki/Graph_theory

http://en.wikipedia.org/wiki/Euler_trail

an ideal sins map would be an Eulerian Graph and you could make a trade route with (n-1) lanes in it, where n is the number of grav wells.