Do you say a straight line? this is not possible in Dgraph. Your map lib/framework just needs the points and then it decides which route it gives you if it needs to generate a route with distance. And straight lines are not so useful. Even for helicopters 
There is also no sorting for the Geo type. Even if there was a way (it could be introduced) to tell the distance between two points, there would be no way to do sorting.
If we see more interest on this topic, we can make it a feature request in the future.