WIP Remove d3 dependency

Part of #1510 but I could do with some help from someone that knows how the original code was supposed to work.

- [x] Remove `_colorize`.
- [ ] Rework `@html` template to use just html.
> What's confusing though

Admittedly, pretty much everything. But your particular confusion here was because there are twice as many stops as indexes. Where I misspoke was that there are actually more indexes than "buckets" (ones that can actually have annotations) because there are a few empty buckets at the terminals.