Add Species Habitat Dataset for Faceted Map Examples

[dsmedia]

This proposal is for adding both a new dataset and its generation script to vega-datasets to enable categorical faceted map examples. This addresses a gap identified in vega/altair#1711 where we currently lack examples suitable for categorical faceting across geographic regions (unlike existing examples of temporal/ordinal faceting like income by state groups).

Dataset Description

The dataset will contain county-level habitat distribution data derived from USGS Gap Analysis Project (GAP) species habitat maps:

• Format: CSV file (consider JSON or alternative formats?)
• Content: County-level habitat percentages for multiple species
• Size: ~3000 counties × 4 species
• Compatibility: Works with existing US county topology file us-10m.json in vega-datasets
• Fields: county_id, species, pct (percentage of suitable habitat)

Sample data structure:

county_id,species,pct
53000,robin,0.138
53073,robin,0.710
30105,robin,0.160

To do:

• Create generation script - Uses exactextract for efficient processing (~20 minutes total) thank you @mattijn + @dangotbanned
• Generate sample dataset
• Share visualizations with different species combinations to find the preferred mix

Follow-ups

• add example to altair (and vega-lite?) gallery

Benefits

1. Fills gap in example datasets for categorical faceted maps
2. Uses public domain data with clear categorical groups
3. Works with existing topology data

Tracking

(Roadmap issue) that may be addressed by fix: facet fails on geoshape vega-lite#9292

• facet fails on geoshape vega-lite#3729

[domoritz]

Great. If it's for Altair only, an option could also be to use parquet. Otherwise CSV or Arrow makes sense.

[dangotbanned]

Great. If it's for Altair only, an option could also be to use parquet. Otherwise CSV or Arrow makes sense.

I've updated the description to get the tracking working for (vega/vega-lite#3729).

@domoritz I know that you're quite busy with (vega/vega#3990), so I'll just provide a summary of what I understand.
Maybe someone else with the right domain knowledge can help get that PR over the line.

@mattijn's example in (vega/altair#1711 (comment)) would only work for altair.
It works around a known bug with facet and {"mark": "geoshape"} via a loop & concat combo.

The potential fix changes only a single line in vega-lite/src/compile/projection/assemble.ts

The blame shows this code was last modified in (vega/vega-lite#4843) by @jheer (6 years ago).
However it appears that line has been there since the file was introduced in (vega/vega-lite#2734) by @willium (7 years ago).