The above example is intended to demonstrate adding a hyperlink to an object (the rectangle) and instructing the text object to be transparent to the pointer so that the link on the rectangle is recognised 'through' the text. This is the example code for a section on adding links to d3.js objects in the book D3 Tips and Tricks
https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.17/d3.min.js