This is designed to demonstrate generating tool tips with an HTML link in them. The graph is based on a simple line graph per the book D3 Tips and Tricks and was generated in response to a question on the d3noob.org blog.
The main differences with this example are the addition of the <a> tags in the appropriate places as commented, the removal of the pointer-events: none attribute in the `style section and the slight amending of the transitions of the html divs so that the tool tips stay in place while the mouse moves off the dot.