Display previous months on the X axis

You can create another measure to establish a virtual relationship between the disconnected table and the related dates table using TREATAS.

Agg selected month =
CALCULATE (
    [Sum of Value],
    -- Applies the [Sum of Value] measure within the specified filter context.
    KEEPFILTERS (
        -- Ensures existing filters on the DatesTable remain in place while applying the TREATAS filter.
        TREATAS (
            VALUES ( DisconnectedDatesTable[Month and Year] ),
            -- Retrieves unique values from the DisconnectedDatesTable[Month and Year].
            DatesTable[Month and Year] -- Maps the values from the disconnected table to the active DatesTable[Month and Year] column.
        )
    )
)