Explore dropping Pandas requirement for Plotly Express

[ndrezn]

In #4790, nearly all Pandas functionality is removed from Plotly.py, except some of our trendine functions.

Narwhals does not yet support rolling, expanding nor ewm, and until that support is added, we need to use Pandas to calculate trendlines.

Those functions are on the Narwhals roadmap, and we should have a quick follow-up in Plotly.py to implement those changes after the first set of Narwhals changes are made and the good folks at Narwhals release that update 🙂 .

See: https://github.com/plotly/plotly.py/pull/4790/files/6676061152527160b44dbf67ecf6b89e46eef3b2#diff-e667a84629ee8d2879cf22a88c8ff80bcfec1c35be69ab9f50be5938c7089ddaR114-R129

[FBruzzesi]

Commenting here to brainstorm how to eventually achieve this.

Currently it is possible to pass trendline_options to forward kwargs to the underlying (pandas) trendline function.

If we were to implement rolling and ewm in narwhals, they would have (a subset of) the polars respective method signature. This would result in a breaking change in plotly, as the pandas kwargs are different.

I think the most reasonable option would be to remap pandas arguments to narwhals with a deprecation warning for a first period, and eventually remove support for pandas specific.

Thoughts on it?

cc: @MarcoGorelli