Change the API to be similar to the Plot API for v2

[mauriciopoppe]

Check out the examples in https://observablehq.com/plot/getting-started, the API is very similar to function-plot.

• Refactor graph types to classes. #355
• Refactor tip implementation to be another type of datum #356
• Refactor annotations to be another type of datum. #358

[mauriciopoppe]

The ability to send graphs as objects was added in #355