I was in the same situation. I can do back-end web programming, and some lower level things in C or mobile apps.

Right now, though, d3.js gives me 80% of my income. It's something I like, and I noticed a lot of demand, so I sort of happened naturally. Focusing in a single area has the advantage that you can become really efficient at it, much more than if you hop between completely different projects every two months.

I still have a bunch of side projects in every language under the sun, because that's what I like, and because the market won't stay the same forever, but focusing professionally on only one area (that you reasonably enjoy) for a time simplifies marketing a lot.