Add choropleth support to GeoAxes

[cvanelteren]

Add a first-class GeoAxes.choropleth API for polygon geometries and country-keyed values, including colorbar integration, missing-value styling, documentation examples, and targeted geographic tests. Closes#622.

[Copilot]

Pull request overview

Adds a first-class GeoAxes.choropleth API for coloring polygon geometries (including country-keyed inputs) and integrates it with UltraPlot’s colormap/colorbar pipeline, along with docs examples and geographic tests (closes #622).

Changes:

• Implement GeoAxes.choropleth() plus supporting geometry projection/path helpers in ultraplot/axes/geo.py.
• Add targeted choropleth tests for collections, missing-value styling, country resolution, zorder, and validation errors.
• Add documentation examples demonstrating polygon- and country-based choropleths.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
ultraplot/axes/geo.py	Introduces GeoAxes.choropleth() and helper utilities for projecting shapely polygon rings into backend coordinates and producing PatchCollections.
ultraplot/tests/test_geographic.py	Adds unit tests covering choropleth rendering behavior, country-identifier resolution, zorder defaults, and input validation.
docs/projections.py	Adds a new projections guide snippet demonstrating polygon choropleth usage with colorbar formatting.
docs/examples/geo/04_choropleth.py	Adds a standalone documentation example for country-keyed choropleths with missing-value styling.

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[gepcel]

From the example, the edgecolor="white" seems having no effect.

[cvanelteren]

That is a z-order issue let me fix that. The choropleth shapes are drawn on top of the existing geometries.

[cvanelteren]

should be good