There are some issues with the opacity when a new event is fired during the transition (if you move the mouse from one circle to the other). This could be solved if you also add a transition of the same duration to the this element on setting the opacity to 1. I don't like too much the fact of adding the property index to the dataset, but I haven't found any better way to do that! Anyway, good job!

The zoom feature only works on the X-axis. It could also work on Y-axis. And there's an overlap with the original axis and the one after zooming. See the class notebook on how to avoid the append("g") that causes this effect: https://observablehq.com/d/9d6f7455066713a2?collection=@antonbardera/2022-23-mvtec-data-visualization-programming-module#cell-18