Allow chart plugins to add more options to the base settings element

This is looking good. Can I ask you to update the issue summary, @nikathone?

Nevermind, @nikathone, sparing you the documentation!

Looking good from my end. @nikathone, can you review?

Looks good to me. Thanks for the help with the issue description update and the code to that use the new API.

Thanks, @nikathone!