plots: allow to modify any (Vega) prop

[jorgeorpinel]

Could plots modify behave a bit more like config where it accepts a key pair as arguments? Instead of relying on specific options like -x and --title to set the corresponding Vega properties.

All of these could be valid keys: https://vega.github.io/vega/docs/specification/ although validating the values could be tricky...

[pared]

We could actually go with dictionary/json update. When we were talking about smoothing plots I mentioned that approach.
We could let user provide his own "modification" that would be applied only to particular plot - so the end plot would be a sum of template + modification applied with dvc plots modify (or even yaml written in dvc.yaml).

[pared]

As to veryfing, maybe we could make use of vega schema, but its quite big, not sure if its worth it:
https://raw.githubusercontent.com/vega/schema/master/vega-lite/v4.17.0.json

[skshetry]

@pared, datamodel-code-generator can generate the pydantic model from the JSONSchema: https://pydantic-docs.helpmanual.io/datamodel_code_generator. That might be useful.

[dberenbaum]

Probably not something we are likely to prioritize. Closing but happy to revisit in the future.