GraphView is not meant for usage outside VFXGraph and ShaderGraph for now as it involves a lot of internal APIs that we need to formalize before exposing them.
In order to do custom ones you would need to use the MeshAPI of the visuallElements to generate the geometry.
I would do a separate element for the control point and the line rendering.
You can have an overview of how to use the mesh API here: Draw a line from A to B