Convex hull mark?

[mbostock]

[Fil]

Half-related: Density Contours Mark (#63).

[Fil]

A plot plugin that implements a convex hull: https://observablehq.com/@fil/convex-hull-mark

The hull is computed in screen space, not data space, which is probably what we want in any case.

[Fil]

Related: #812 (since d3.Delaunay computes the hull). We could also do an ellipse (i.e. the smallest ellipse containing the data) or a superellipse, cf. geom_ellipse in ggplot2.

Both of these can be done in data space (as transforms); contrast with Delaunay and Voronoi, which should be done as layouts/initializers (#801), since the x and y dimensions are not separable.

[mbostock]

Implemented in #917.