Redo activity / installs chart in Plottable

![image](https://cloud.githubusercontent.com/assets/1400023/3742580/4ca26a84-176b-11e4-9741-c5cdf45bd843.png)
Features:
* Shows 7-day moving average rather than daily to give clearer view into trend
* Shows npm installations as a line chart, with a second axis
* Cleaner time axis
* Intelligent resize behavior
Is it possible to make the 3 legend labels in a row and centered? so the legend takes less vertical space. not sure if it's an edit here or in css