Hola, you may want to pin this to d3@5 because it will fail when d3@6 is published. I made a fork that works with d3@6 (d3-group), but I'm not sure if it's the simplest way to explain d3.group—but it was a good exercise :) https://observablehq.com/compare/b30b92719dfef419@102...5135bc575e64badb@149 (another possibility is to require("d3@6", "d3-collection") and keep d3.nest)

Thanks a lot! I like the idea of switching the example to d3-group, I'll merge your changes. I agree this isn't the best way to explain d3 group or nest, but I have find many times that people struggle using d3 stack because their data isn't in the proper format. That's why I made this. By the way, we are doing a course redesign now which includes recordings that are going to be used for many years. It would be good to do everything with v6, can we talk about this?