Add id, class, and alt attributes to individual points

Hello,

I wanted to bind click events on points (e.g., to do an Ajax call). By adding a bit of metadata (i.e., id, class, and alt) to individual points we offer more flexibility to interact with them in pure JavaScript. Is that against the Rickshaw design principles?

I only added it to the scatterplot as I could not figure a way to have it available to all renderers without duplicating code. Please let me know if there is a way to do so.

Thanks,
Georges

P.S. Great job on that toolkit!