We were using flot when I started. As soon as I found out about protovis I redid all our charts (and boy was everyone happy with that improvement). Shortly after that, d3 came out, in which case I ported all the protovis charts to d3. NVD3 is my second go at a collection of charts for d3.js... it was re-done from the ground up, and hopefully is much more true to d3's style than my previous attempt.
I might have used protovis for my first iteration, if not for the fact that I discovered it just as d3 was introduced :)
I'm normally a server guy, but needed a good dashboard for our OpenTSDB metrics project, so I started diving into client-side JS -- going whole-hog on d3 without a good chart lib on top of it seemed to be a bit too steep of a learning curve. But I do love the possibility of open-ended charts, where we can dip into the power of d3 at will when we need something more custom.
Like DiabloD3 commented below, I feel like doing an end-zone victory dance :). Nice work.
