Ideally, find some kind of community where you can discuss with and help each other out regularly, but I don't really know of such a place, and it can be hard to find someone to do this with physically outside Paris, Berlin, NYC, and SF.
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.
Find some comparison tutorials between canvas and either of these, to see just how much more labor-intensive canvas tends to be. Then maybe make a decision as to what to dive deep into first.
If I could recommend a way to learn d3.js:
1. Read Scott's book (and code the examples).
2. Think of some simple charts you want to make (and code them).
3. Use #d3.js@freenode, Stack Overflow, and see if some of the articles at http://www.d3noob.org/ fit what you're trying to do.
4. Go to http://bl.ocks.org/mbostock, be inspired, fork the code, and apply it to your own idea.
Repeat #3.
Ideally, find some kind of community where you can discuss with and help each other out regularly, but I don't really know of such a place, and it can be hard to find someone to do this with physically outside Paris, Berlin, NYC, and SF.