So bit of a head-scratcher this one!...
So one(and I think the only!) way to do this is to envoke another Table Calc, and nest the current MAT calculation in it
So I created this calc....
[MAT - Window Sum]
So when we set up the table calulation we can used the nested feature to run MAT and MAT - Window Sum different ways (one over date, and one over company)
So the MAT is run by date as before and MAT - Window Sum by Company
This now gives us the running_total 12 month SUM of each company, so we can just divide one by the other to get the Share of Sales.
Hope that helps, and makes sense.
Dynamic MAT share example.twbx 95.4 KB
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It worked beautifully! Many thanks SImon!