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.
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.