Actual Yaw deviating from desired yaw ArduPlane

Hi everyone

I have this graph of actual yaw vs desired and I am not sure whats causing the trouble and I am trying to tune it as described:

https://ardupilot.org/plane/docs/roll-pitch-controller-tuning.html

any suggestions looking at the graph??

There is no concept of desired yaw in forward flight, hence it is always zero.

Hi Peter thank you so much for the information