A solid JS library for statistical computing is definitely long overdue. After skimming through it, JStat seems to only show some trivial examples, and it doesn't seem to be designed for being extended.

What I would really like to see some day is a JS library that extends D3 for graphics and controlling DOM events, that has a good standard library for common statistics (and that is easy to extend), and ideally a possibility to delegate computationally intensive tasks to C (perhaps FFI through Node.js?)