How to: value(now) - value(some time ago) to see Difference over time?

Hello, I asked that in the Grafana Forum allready, but with no result. Maybe somebody here knows how to do that:

I have that Graph:
SELECT mean(“value”) FROM “Volumen_OelTank” WHERE $timeFilter GROUP BY time($interval) fill(null)

which shows the Volume of my tank. Now I’d like to see how much I use
like every day or every week. So I like to show the difference between a
value and the value one day/7days ago.
How do I have to formulate that?

I use Influxdb as input data.
Thanks for your help,

You should use persistence and can calculate with extra extensions. The demo site has a working example for minimum and maximum temperature.

I’m using persistance, that’s the way the Data gets into the influxdb.
Yes, I could calculate the difference of the actual value and a value from last week within OH, but the question is, if grafana can do that itself. It has to get two values from the influxdb and substract them. But how can this be done?