Spent a bit of time working on a d3js version. It's super ugly because I just hacked it together while learning how d3js works; I'm sure someone will come along and do it much better ;)

http://bl.ocks.org/3815828

I imagine it's only a matter of time before Mr Bostock catches on to this and creates an example: http://bl.ocks.org/mbostock.

d3.js should probably have you covered.

Such lib is not hard to write. Somewhat limiting but simple version could just be a Canvas API wrapper that breaks down lines and curves into smaller lines then jiggle the points randomly before rendering the points out using Catmull-Rom to Bezier spline conversion.

Complicated part is in intelligent fragmentation and jiggling based on hand velocity and hand muscle modeling but it's pretty easy to get decent result. Another technique to combine this with is drawing each path more than once but, like bold fonts, result is not as generally useful.

I wrote up a quick example using d3, adding some jitter to the graph line gives it the hand-drawn look, plus used a font from the xkcd forums

http://bl.ocks.org/3816112

Not a library, and not for graphing, but somewhat inspirational: http://hakim.se/experiments/html5/sketch/