make error messages more helpful when choropleth features and data don't match

I teach a class for students that are new to coding. We use plotly, and I'm happy to have a package that gets students good-looking graphs in so little code.

I introduce plotly's choropleth maps, and have repeatedly run into the issue where the students get confused by what the featureidkey is referring to and how that relates to the locations. I do my best to explain this, but they are (understandably) confused working across CSVs and GeoJSON. One student just hit an issue where they were trying to use FIPS codes, but their locations column had leading whitespace. When the rows are unable to be matched to a feature, it fails silently.

My feature request is to provide a warning when the featureidkey and locations have zero matches.

I haven't contributed to the plotly codebase before, but I'd be open to doing so if the maintainers are open to receiving this. Presumably the place to add this check is one of the validators?

Thanks!

Start in packages/python/plotly/plotly/validators/choropleth, which the issue identifies as a possible location for the check. Reproduce a choropleth using GeoJSON feature identifiers and locations with no matches, including leading whitespace in FIPS codes. Done means the zero-match case produces a clear warning instead of failing silently.