Accumulated data per interval and data downsampling

Hello Alex,

let me begin with a thank you for writing this app. It looks promising to me. What I would like to suggest as features are:
* Display accumulated data
* E. g. in a widget were one can edit the title and the formular
* This would be especially useful for measuring the rain amount were commonly 24h/12h/6h values are more interesting.
* Data downsampling
* As the data is obviously stored in a RDBMS and not a TSDB I think it could be useful to have rules like in RRDtool to downsample data older than time x to prevent the DB from getting too heavy.

Greetings
Eike