Total of duration actually showing in a report in summary bar or other chart

Hi!
We want to create a report to send to our clients, as a basis for invoicing.

It should ideally be able to have different rounding rules for reported time on different tasks, since some tasks are billed per half hour, where others may be rounded to the nearest 5 or 15 minutes.

We can solve this by using tags and filtering, and make separate charts for the different rules.

The problem however is the summary bar, or a chart with the same function. As long as we have time reports with different rounding rules it will never be correct.

So, is there some clever way to, inside the report, get a summary of the total duration actually shown in the individual charts in the report?

Thankful for any help! :folded_hands: