Andrew Catton explained how the things are:
"We don't have this at present (honestly, it hadn't yet come up in talking to your users). One variation on this that has come up, and we do support, is the ability to group dated entries by month, with running subtotals.
If we do see demand for this, we'll certainly considering finding a way to add it.."
My data looked like this:
And the report with group entries by month, with running subtotals:
So I will keep you informed if anything changes. I guess sooner or later Dabble DB developers will find this issue worth a solution and show the way to calculate a running total. We will see.