Power Query - how we create a measure based on a dataset while union it back to the original dataset

Peter_Price

And you can also use a DAX Calculated Table as well.

Assuming your Table Name is Table1.

From Modelling Tab >> New Table

File attached as well

Calculated Table =
VAR Rows_I_need =
    GENERATE (
        SELECTCOLUMNS ( VALUES ( Table1[Category] ), "Mycategory", [Category] ),
        CALCULATETABLE ( TOPN ( 1, Table1, [Date], DESC ) )
    )
VAR Add_Volumn_Difference =
    ADDCOLUMNS (
        Rows_I_need,
        "Difference", [Volume]
            - CALCULATE (
                SUM ( Table1[Volume] ),
                TOPN (
                    1,
                    FILTER (
                        Table1,
                        Table1[Category] = [Mycategory]
                            && Table1[Date] < EARLIER ( [Date] )
                    ),
                    [Date], DESC
                )
            )
    )
VAR ComparisonTable =
    SELECTCOLUMNS (
        Add_Volumn_Difference,
        "Date", [Date],
        "Category", [Category],
        "Calc Type", "Comparison",
        "Volumn", [Difference]
    )
RETURN
    UNION ( Table1, ComparisonTable )