Just to add that I'm experiencing the exact same issue... and thought the same "it must be something stupid I'm doing". Tried fresh install, issue remains: orbit trajectories only show at low zoom levels, as soon as I zoom out they disappear. Running KSP 0.90 on Lubuntu 14:10 EDIT: I found that changing my desktop resolution made the lines reappear... guess there must be something screwy with resolutions. Perhaps the relationship between monitor/OS desktop setting/KSP setting maybe. Anyways, all good here now.