Redesign register_facet_classes plugin hook

[simonw]

Nothing uses this plugin hook yet, so the design is not yet proven.

I'm going to build a real plugin against it and use that process to inform any design changes that may need to be made.

I'll add a warning about this to the documentation.

[simonw]

#427 had a bunch of ambitious plans for faceting that I haven't realized yet:

Think of all of the potential kinds of facets:

• ?_facet_array=tags where tags is a JSON array of values
• _facet_date=datetimecol - faceted by date part of a datetime
• _facet_bins=numeric_column - can I do some kind of fancy binning here? Might need to take an argument
• ?_facet_bins=numeric_column:5 - could be a way to take an argument. We’ll ignore columns with a : in their name.
• ?_facet_json=jsoncol:jsonpath - could use a JSON path to extract out something to facet on?
• ?_facet_percentile=numericcolumn - could this work?
• ?_facet_function=column:sqlfunctionname - maybe this could be interesting? Would allow for e.g. facet by soundex
• ?_facet_prefix=column:prefix - facet by terms but only if they start with a specific prefix
• ?_facet_substring=column:3,6 - facet by a substr(column, 3, 6)

[mroswell]

Related: #1285, including arguments for natural breaks, equal interval, etc. modeled after choropleth map legends.

[simonw]

As of today, 16 December 2021, I'm still not seeing any evidence that anyone is using this hook (yet) according to GitHub code search: https://cs.github.com/?scopeName=All+repos&scope=&q=register_facet_classes%20-repo%3Asimonw%2Fdatasette