I think I found the solution for you. I just created 2 calculated fields for the value of A and B at record level, without attr and sum functions:
IF [Instrument]="A" THEN [Value] ELSE 0 END
if attr([Instrument]) = "A" then SUM(Value) else 0 end
You can then put sum(A Value) and sum(B Value) on the columns and rows shelf and use [measure date] as detail.
Below is a screenshot of the resulting plot and a packaged workbook is attached.
Let me know if this is the result you were looking for!
ScatterPlot V2.twbx 20.2 KB
Awesome! that's exactly what I was looking for. Thanks much!