JavaScript Library for HTML5 canvas based heatmaps

I've been using this for an upcoming project of mine, for static heatmaps, really simple to use.

I do wish you could alter the shape of what's plotted - at the moment it's just circles of different intensity layered on top of one another - it would be really useful to be able to draw a shape of something else (like a line or area).

I'm just downloading it now. Looks great! Can this be used to track usage and send it data back to the server without drawing any visuals?

How do you get the right x-y coordinate on a page when its height or width can change (i.e. responsive)?