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) but could not find a convenient way to do so. By adding a bit of metadata (i.e., id, class, and alt) to individual points we have more flexibility to interact with them in pure JavaScript. Is that against the Rickshaw design principles?

In other words, 
{ "x": 45, "y": 10, __"id": 42, "class": "my-item", "alt": "My Item"__ }
turns into: 
\<circle cx="5" cy="10" r="4" fill="#c05020" __id="42" class="my-item" alt="My Item"__\>\</circle\>.

I only added it to the scatterplot renderer as I could not figure a way to make 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!