isobands?

[Fil]

an isoband is the region between two thresholds; we offer them as an iterator in d3-tricontour, and they can be useful (e.g. riatelab/bertin#111; https://observablehq.com/@fil/tricontour-isobands; https://observablehq.com/@fil/spherical-isobands).

Algorithm: iterate over the contours in pairs, consider the upper's rings as holes to be distributed in the lower's rings.