Overlapping Voronoi polygons

When I tried to create a Voronoi diagram from a gridded dataset of 4 points (±90, ±45) using geoVoronoi(), the cells seem to overlap. @Fil has graciously pointed out that this is because the polygons have some 180° edges but no intermediate points.

Here’s a minimal, reproducible example: https://observablehq.com/d/1433b09bb55a4807. Ideally, each Voronoi cell should be colored either orange or blue. Would love for a way to fix this. Thanks!

[Fil]

With “2 columns”, the points are in fact collinear (all belong to the great circle that passes through the poles and [-90;0]). It’s one of these situations where the (planar) Delaunay triangulation also tends to fail.

A “solution” in that case can be to introduce a supplementary point that is very close to the first point, but not aligned on the same grand circle. (It’s frustrating that the code can’t compute this case on its own, tho.)

[Fil]

I believe this shows two distinct issues:

1. when we have 2 columns (and whatever the number of rows), geoVoronoi returns ill-defined polygons.

2. when you request more than two columns, the polygons are OK, but d3.geoEquirectangular (and, it seems, all the projections that rely on antimeridian clipping) fail to display them properly. (You can see that by switching to d3.geoAzimuthalEqualArea in your notebook.)