This is a follow on from the simple line graph written with d3.js v5 and based on @mbostock’s example here.
It impliments a simple tooltip which I find particularly neat. This implimentation is designed as a simple example which will be built on to create a slightly more complex example.
This graph is part of the code samples for the update to the book D3 Tips and Tricks to version 5 of d3.js.