you are stumping me on this one...
lets try 2 formulas
lets convert the date field to the first of the month (call it "DateConverted")
dateserial(year(table.date),month(table.date),1)
Let's convert the tabledescription to another text (call it "NewText")
if table.tabledescription = "table1" then "SALE" else
if table.tabledescription = "table2" then "RETURN" else 'ERROR"
in your chart on change of use @DateCOnverted set to monthly and then @NewText under that
in the Show Values continue to use Struc_ID set to count
Does this give you anything better?
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