A new "npm run" target for building custom bundles

[nicolaskruchten]

I'd like a straightforward set of instructions that a user can use to:

1. Create my-custom-bundle.js based on something like lib/index-basic.js
2. Run npm run build-custom-bundle my-custom-bundle.js and it would produce my-custom-bundle.min.js or whatever

This is needed so as to be able to provide clear instructions to Dash developers that want to make their own bundles :)

[nicolaskruchten]

The custom-bundle build instructions should be documented in https://github.com/plotly/plotly.js/blob/master/BUILDING.md, as this is what I'm linking to from dash-docs for "custom-built bundle".

[archmoj]

Right now scatter trace (puls cartesian subplots) are added to every bundle by default.
Do you think it should not be added by default for custom bundles?

[alexcjohnson]

If a bundle without scatter works, then yes. But I think it was put in core originally because otherwise something or other would break.