Issue with 12 Month Moving Average Calculation - Need Help

Hi,

i am facing issue while calculating 12 month moving average.  I am using below formulas -

Number of periods =
CALCULATE (
    CALCULATE ( COUNTROWS ( VALUES ( DimDate[MonthName] ) )),
    DATESBETWEEN (
        DimDate[Date],
        NEXTDAY ( SAMEPERIODLASTYEAR ( LASTDATE ( DimDate[Date] ) ) ),
        LASTDATE ( DimDate[Date] )
    )
)

--------------------------------------------------------------------------------------

12 Month Sales Moving Average =
CALCULATE (
    [TotalVolume],
    DATESBETWEEN (
        DimDate[Date],
        NEXTDAY ( SAMEPERIODLASTYEAR ( LASTDATE ( DimDate[Date].[Date] ) ) ),
        LASTDATE ( DimDate[Date].[Date] )
    )
)
    / [Number of periods]

-------------------------------------------

the above formula is working correctly. But there is one issue. Sales table is missing some months transaction. Like there is no transaction for October, November & December 2018 month but still graph shows [12 Month Moving Average] for these months.

Similarly few months are missing from January to September also but value is coming.

My requirement is that graph should show [12 Month Moving Average] for the months for which transactions available.

DimDate is monthly & generated for full 2017 & 2018 year. All months are covered.

Sales table has sales data but some months are missing.

Now you see on above picture some value is coming as Blank & also value for October, November & Deceber 2018. But there is no transaction for these months.

Please suggest how to fix this issue.