have an item that is collecting data for some 20-40 minutes. click on "one week graph" for it in data overview. notice how graph shows 7 days. drag-zoom to some 5 days - notice how the area says "2h", and then zooms not to the period we chose, but 2 hours and some.

this is probably because we try to limit the maximum time period user can select if we think there's not enough data.
i'd suggest getting rid of this restriction, i don't see a good enough reason to keep it.