Add new row with accumulate in Matrix

Hi marcio_fornari ,

I updated your sample pbix file(see attachment), please check whether that is what you want.

1. Create a calculated table with below formula

Table = UNION(VALUES('Sales'[Product]),ROW("Product","XMonth Accumulate"))

2. Create two measures as below to get the sum of sales

Measure = 
VAR _selmonthname =
    SELECTEDVALUE ( 'Date'[MontName] )
VAR _selmonthnum =
    CALCULATE (
        MAX ( 'Date'[MontNumber] ),
        FILTER ( 'Date', 'Date'[MontName] = _selmonthname )
    )
RETURN
    SWITCH (
        SELECTEDVALUE ( 'Table'[Product] ),
        "XMonth Accumulate",
            SUMX (
                FILTER (
                    ALLSELECTED ( 'Sales' ),
                    MONTH ( 'Sales'[Date] ) <= VALUE ( _selmonthnum )
                ),
                [Sales (Selected Month)]
            ),
        SUMX (
            FILTER ( 'Sales', 'Sales'[Product] = SELECTEDVALUE ( 'Table'[Product] ) ),
            [Sales (Selected Month)]
        )
    )

Month Accumulate = 
IF (
    ISINSCOPE ( 'Table'[Product] ),
    SUMX ( VALUES ( 'Table'[Product] ), [Measure] ),
    [Sales (Selected Month)]
)

3. Create a matrix visual (Rows: field Product of Table   Column: field MonthName in Date table  Values: [Month Accumulate] )

Best Regards