What are graphs like these called? And how do I get started on building them with D3.js?

I want to build graphs like these for my upcoming project on D3.js but I have no idea how and where to begin. It looks like they're just simple line charts but the interactivity confuses me. I tried looking for documentation on these sort of graphs but I couldn't find any. If anyone knows how or where I could go to get started, it'd be greatly appreciated. Cheers!