Desired course NMEA datagram (which datagrams are supported?)

[nohal]

As I understand you, you don't need to look for any NMEA datagram as all you want to work with is contained in fields 11 and 13 of the APB sentence OpenCPN sends to the autopilot.

You also don't need to develop anything to visualize it as the draw plugin EBL with fixed end position draws exactly what you are looking for.

[rgleason]

Tactics_pi might have something like this.