Is Unity 5.6 NavMesh Data Available to Gizmos/Editor?

I have a script from Dungeon Architect that basically draws Gizmo lines along all connections between neighboring waypoints, which is nice. However, I'd like to draw a secondary line the follow the lowest cost path on the NavMesh between two points (in the scene view as a gizmo, preferably). There's a sample I'm trying to work from here:

https://docs.unity3d.com/ScriptReference/AI.NavMesh.CalculatePath.html

However, if I debug through it, path never gets any values. I created agents on the objects at the two points, thinking maybe that would make a difference (I'm not sure why I thought that). Does this script need to be attached to one of those points? With a NavAgent? Or am I missing something completely obvious?