coerce timestamps so graphs from disparate sources line up

When timestamps from different sources don't line up well, things like composites and aggregation functions don't work so well in Librato. Here's a screenshot of a metric set to display "sum of derivative" on a counter before and after:

![image](https://cloud.githubusercontent.com/assets/22596/4911610/6c303eb8-6490-11e4-8a87-5b52f3b48185.png)

Hooray :beers: