Balance Sheet and cumulative sums

[kruel830]

Due to outsourcing of accounting, I have to rearrange Qlik Sense reporting.

The most problematic thing is  The Balance Sheet. Before the change I had cumulative sums for each account for every month from our ERP. Now I have opening balance for each account, but after that records only tell the change in that balance.

There is no problem to refer single month figures, set analysis goes like:
sum({<Year={$(=max(Year)},MonthNum={"<=$(=max(MonthNum))"},YearMonth=>} [Sum Eur] )
or
Sum({Date={">=$(vStartOfSelectedYear)<=$(vEndOfSelectedMonth)"},Month=, MonthYear=,Year=>} [Sum Eur])

The issue arises when I want to show e.g. value of inventory for last 12 months. The graph looks like this:

I need to create cumulative sums for each account (and account group) for each month in load script, but I don't have any clue how to do it. Any tips?