YTD Average & Average Change

Hi,

According to your description, i create a sample to test:

Please try to create a column header first:

ColumnHeader = 
    UNION (
        ADDCOLUMNS (
            DISTINCT ( 'Table'[Date].[MonthNo] ),
            "Month", FORMAT ( DATE ( 2020, [Date].[MonthNo], 1 ), "MMM" )
        ),
        DATATABLE ( "MonthNo", INTEGER, "Month", STRING, { { 13, "Change from Prior Month" } } ),
        DATATABLE ( "MonthNo", INTEGER, "Month", STRING, { { 14, "YTD Average" } } ),
        DATATABLE ( "MonthNo", INTEGER, "Month", STRING, { { 15, "Change from Prior Average" } } )
    )

It shows:

Then create a measure:

Measure = 
VAR MaxMonth =
    MONTH ( MAX ( 'Table'[Date] ) )
VAR LastMonth =
    MONTH ( MAX ( 'Table'[Date] ) ) - 1
RETURN
    SWITCH (
        MAX ( ColumnHeader[Month] ),
        "Jan", CALCULATE (
            SUM ( 'Table'[Values] ),
            FILTER ( 'Table', MONTH ( 'Table'[Date] ) = 1 )
        ),
        "Feb", CALCULATE (
            SUM ( 'Table'[Values] ),
            FILTER ( 'Table', MONTH ( 'Table'[Date] ) = 2 )
        ),
        "Mar", CALCULATE (
            SUM ( 'Table'[Values] ),
            FILTER ( 'Table', MONTH ( 'Table'[Date] ) = 3 )
        ),
        "Change from Prior Month", CALCULATE (
            SUM ( 'Table'[Values] ),
            FILTER ( 'Table', MONTH ( 'Table'[Date] ) = MaxMonth )
        )
            - CALCULATE (
                SUM ( 'Table'[Values] ),
                FILTER ( 'Table', MONTH ( 'Table'[Date] ) = LastMonth )
            ),
        "YTD Average", FIXED ( AVERAGE ( 'Table'[Values] ), 2 ),
        "Change from Prior Average", FIXED (
            AVERAGE ( 'Table'[Values] )
                - CALCULATE (
                    AVERAGE ( 'Table'[Values] ),
                    FILTER ( 'Table', MONTH ( 'Table'[Date] ) <> MaxMonth )
                ),
            2
        )
    )

Apply a filter to matrix visual such as [Category]="AAA", the result shows:

Here is my test pbix file:

pbix 

Hope this helps.

Best Regards,

Giotto Zhi