looking at this again, I think you've got it working :-)

some of the long-haul routes are showing up as curved, although it's difficult to see - there are a lot of short-haul routes in there. 

Reducing line opacity can sometimes help with this sort of visualisation (or making shorter routes more transparent)

When zoomed far in, great circles tend to look linear - you'll typically only see curvature over hundreds or thousands of Kilometers. If you zoom out, the curves should appear as you expected.