My assumption is that the in-game plotting uses the server because the navmap for the ship is stored on the server, ie a jumphole is only added to the navmap when the server sees you are in scanner distance of it (planets and some other things are handled differently). I ass-u-me that the navigation system was designed to use this information, and since the data is stored on the server, the calculations would have to be performed by the server too. They are not currently linked though; the server will calculate a path based on jump gates that you have not visited, while ignoring jump holes that you have visited.
Also, AD was playing with something a little while ago that was supposed to incorporate that information but was badly bugged
So it is definitely server-side but dunno anything about how it is implemented