text: Title: [FEA] typecheck invalid api=... value
  Body: **Is your feature request related to a problem? Please describe.**
  When mistakingly writing `register(api='3')` vs `api=3`, error happened at `.plot()` and in a surprising way. 
  
  **Describe the solution you'd like**
  Would have been better for `register()` to throw a type error, and `plot()` errors that nothing set.
  
  