Have tooltip show date ranges for aggregated data

[pluehne]

Sometimes, when looking at weekly-aggregated charts, it confuses me to see the date of the first day of the in the tooltips when hovering data points. Users could also be confused, because it’s not clear whether the shown value represents a single date or an entire week (and where the week starts and ends is also undocumented).

This could be addressed by having Chart.js show the respective date range in the tooltips. For data series that have been aggregated by first and last (which select only the chronologically first/last within each period), the tooltip should still show the respective (single) date. However, for data aggregated with sum, mean, med, min, and max, we could show the time range instead.

[pluehne]

@larsxschneider: Working on this now 🙂.