One bizarre workaround right now is to abuse the geographic projections: https://observablehq.com/@observablehq/pie-to-donut-chart
In general, you can do a lot of customization, including plugins (though that’s not well-documented yet), so you could make it work… see https://observablehq.com/@fil/radial-transform, https://observablehq.com/@observablehq/plot-radial-line for other approaches to hacking radial stuff into Plot.
You can do Cartesian tree diagrams in Plot, e.g. https://observablehq.com/@observablehq/plot-tree-flare, https://observablehq.com/@observablehq/plot-custom-tree-layo...
If you want to make a chord diagram like that “Müsli Ingredient Network” example, you could try one of these D3 examples: https://observablehq.com/@d3/chord-diagram, https://observablehq.com/@d3/hierarchical-edge-bundling/2
Is it easy to customize charts? It'd be so nice to use this to do a circular/radial tree/node visualization, e.g. something like this: https://truth-and-beauty.net/projects/muesli-ingredient-netw...