It's my theory these totals are periodically calculated (e.g stored procedure) and then written in an other table. Anyone?

I think that is indeed the case. You can however export the cumulative data from the chart to CSV. This might indicate that these calculated numbers are stored somewhere?

Indeed not for the faint hearted. I was hoping for an other route and avoiding SQL but I guess I'll have to dive deep .
Thanks

I know you can remove a faulty data entry by shift clicking in the graph. However this does not work in cumulative graphs like 'comparing usage.
My raspbery PI was offline for 3 months and after restarting the difference in usage in the past 3 months was added op to a new value.
Is there a way to ...