We haven’t implemented it yet, but that’s what we’re working on right now! There’s a PR up here (we just took a small break to focus on TypeScript declarations and then the new documentation website):

https://github.com/observablehq/plot/pull/1304

In the meantime, you can do lots of interactive stuff with the SVG that Plot generates. There’s a nice tooltip plugin by Mike Freeman here:

https://observablehq.com/@mkfreeman/plot-tooltip

Excellent. If I could recommend some inspiration, NVD3 (no longer maintained) had the exact set of features I found useful, as does billboard.js (https://naver.github.io/billboard.js/demo/#Chart.AreaChart). Namely the ability to hide/focus/show certain series from the legend, showing all series in the interactive tooltip, and (NVD3) highlighting in the tooltip which series you are directly hovered over. These all become helpful when you start charting lots of stuff at once. Thanks for being the bedrock of all of this stuff!

Yes! I've been looking forward to official tooltip support for ages.