Hello, i recently had that issue but with a B to B+.. My issue got resolved, Take some screenshots of your plot and find some other C+ plots that you feel are a lower standard then yours. Once you've done that, I recommend private messaging Christian or Smxthie on discord with your screenshots attached and let them know how you're feeling about it. From experience they listened to me and gave me a B+.
Good luck! C+ really is a decent grade anyway.