It's peculiar how there are a billion beginner-level articles on d3.js, but the people who get stuck usually need something between that and intermediate.
If someone considers writing a d3.js tutorial, I'd urge them to use a real example, since this is where people realize they should use features like line.defined(), figure out how to implement gridlines yourself, and so on.
As great as d3.js, it's really hard to dive into the official wiki/docs as well as hit them for help.
Wow, I've been using d3 for almost a year now and I didn't know that line.defined() existed.
I've made lots of stuff with d3 and never needed grid lines or dealt with undefined line values, so my 'getting your hands dirty' tutorial would probably deal with totally different issues than what would be useful for you. At a certain point, you just have to buckle down and read the documentation - a quick peak at the page for svg.axis shows how to get grid lines extremely easily (http://bl.ocks.org/mbostock/4323929).
Hanging out on the stackoverflow tag is also super helpful. I either know how to answer a question and get some internet points or Lars responds within an hour and I learn something new.
If someone considers writing a d3.js tutorial, I'd urge them to use a real example, since this is where people realize they should use features like line.defined(), figure out how to implement gridlines yourself, and so on.
As great as d3.js, it's really hard to dive into the official wiki/docs as well as hit them for help.