How D3 Selections Work
There’s an easter egg of sorts in that you can run the shown code snippets in the JavaScript console, and the results in the console will match the shown diagrams. This is thanks to careful inclusion of off-screen elements.<p>It would be fun to make an interactive tool that generates the diagram directly from the given selection and data (and probably wouldn’t be too hard to repurpose the existing code). But the JavaScript console and element inspector are so useful on their own for learning D3 + DOM, it’s likely more fruitful to encourage people to use the console directly.