So I finally re-did this tutorial from superHi to try and get my head around using selectors and hooking them up to data. This was one of their tutorials and I modified the css etc a bit for my own little project. If you are looking for some cool D3 tutorials, really recommend superHi! Very good videos and documentation and have also got quite a lot from their Slack community - very helpful!
There's lots to see in here - not just the drop down menus, but also a little cool function called "raise" that helps with occlusion
As ever code can be found here