After contacting grafana directly I received the answer that this is not currently possible. All such computation must be done by the data source, not by grafana.

I've found the answer, we need to utilize the "Script" option. First we set a metric to get sum of apple, then the result of sum of apple is added to doc["orange"].value

In the script i wrote something like this _value + doc["orange"].value

and the value return in the graph is correct! :D

I am guessing that you cannot combine two statements in one query. Try setting the variable in a subquery:

SELECT UNIX_TIMESTAMP(Datum) as time_sec, Datum, Sachkosten as value,
       'Sachkosten' as metric,
       (@csum := @csum + Sachkosten) as cumulative_sum
FROM aufwand CROSS JOIN
     (SELECT @csum := 0) params
ORDER BY Datum ASC;

Or, perhaps this will work:

SELECT UNIX_TIMESTAMP(Datum) as time_sec, Datum, Sachkosten as value,
       'Sachkosten' as metric,
       SUM(Sachkosten) OVER (ORDER BY Datum) as cumulative_sum
FROM aufwand CROSS JOIN
     (SELECT @csum := 0) params
ORDER BY Datum ASC;

Note that in newer versions of MySQL, the ORDER BY may not affect the ordering of the cumulative sum. You would need a subquery with the ORDER BY to ensure that.