This can be achieved using the following expression:
RangeSum( Above(total sum(Sales),0,7) )
Remark:
The Above() expression is assuming that you want to have your table sorted by date in Ascending order (oldest date will be the first date on the table).
In case you want to change the sort Descending (Recent dates comes first) you need to replace the Above() by Below() expression.
Hope it helps!
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