Anyway, I wasn't satisfied with it. It has a lot of limitations - mainly, it can only sketch parabolas. Also, the code behind it is a mess. I thought I could do a better job, and thought it'd be much more useful if it could handle other types of functions.
I spent a few days working on a replacement. Then we went to Australia for two months, and I forgot about it. Then I remembered it today.
So, here it is! Introducing the Algebra Graph Sketcher. I know it's a much more boring name than Parabolator, but I guess my desire to be accurate won out over my desire to be silly this time.
For anyone who's interested, I'm using Vue.js for the control interface, D3.js for the actual graph, and Lodash for... something? I guess this is why you should post about something while you still remember how you made it in the first place.