sparklines or stacked graphs for repo PR trends

[aspiers]

The PR trend graphs are great, but similarly to #14, it would be useful to have a view where they're all stacked together, so we can spot trends across / between all repos. Since there are many repos, there are far too many to put on a single graph. I suggest that sparklines are probably the best way to visualize these. For example, you could use the jQuery Sparklines plugin, or d3.js.

[aspiers]

Here's a tutorial for rendering sparklines via d3. d3 is really awesome; I strongly recommend looking into it :)

[aspiers]

This would be very useful for Jenkins weather reports too.