Hi Simon.
Astrophotography is not easy, and that's the best part of it!
As for your graph, it seems to me that your mount is managing to correct traction for periods of time and then occasionally you get large drifting.
If I was to guess, I would say that there might be some play in your gear; maybe the focuser is not rigid, or tube rings, or maybe you have large backlash? Also, if any of the cords catches up on something it may also stuff up guiding.
Patience and perseverance will be your best allies in solving this issue