DAX for monthly data with missing values

hi, mterry 

Based on my research, you could try this way:

Step1:

Create a dim year month date table or you could just use this formula to create a new dim table

Dim Date = 
SELECTCOLUMNS (
    FILTER ( CALENDAR ( "2018-01-01", "2019-12-31" ), DAY ( [Date] ) = 1 ),
    "Year No", YEAR ( [Date] ),
    "Month No", MONTH ( [Date] ),
    "Year Month", YEAR ( [Date] ) * 100
        + MONTH ( [Date] )
)

Step2:

Add month column for date column in your basic table

Step3:

Use this formula to create a new table

Table = FILTER(GENERATE(Basic,'Dim Date'),Basic[Year]='Dim Date'[Year No])

Step4:

In the new table, create a real outout column

Real Output = IF('Table'[Month]<='Table'[Month No],'Table'[Ending Output],'Table'[Beginning Output])

Step5:

Drag Year month column and real output column from the new table into a visual

Result:

Here is sample pbix file, please try it.

Best Regards,

Lin