Activity plot in dashboard

[jrbourbeau]

@mrocklin last Friday you mentioned some sort of timeseries that was updated regularly would be a nice visual element to have here. I agreed but at the time thought the time axis might seem a little off since the data being generated here has older dates (e.g. 1997 -- though a good year).

Now that we're including processing timestamps in the data filenames, we could pretty easily generate a data coverage or data processing timeseries in the dashboard.