I’m trying to create a fill in between two of these triangles, for example a red fill in between the red and green triangles. Does anybody know how to accomplish this? I have seen another example of a radar graph that I believe does something similar, although I think they create the fill using CSS, not d3…

solved the above problem with mbostock’s answer using radial chart…
Final Result

You don’t need to implement a custom path for this; you can use a d3.svg.area.radial path generator. Here’s an example of a stacked radial area chart which you might use to plot cyclical data:
The implementation is fairly similar to a radar chart, except with a radar chart the scales will be different for each angular point.