d3.json: “Uncaught TypeError: Cannot read property 'children' of undefined”

Question

I am trying out d3 as a tool for representing biological data.

I am trying to open the following example in my chrome browser so that I can understand how it works

Answer 1

In order to get it to work I had to replace the line

d3.json("flare.json", function(error, root) {

with

d3.json("flare.json", function(root) {

I used my intuition here and I still do not understand how it worked on the example website and not mine, or what the true significance of my change in the code is. If you know how to answer this in a more in-depth fashion, please contribute and I will accept your answer.