Dynamic Top N calculation for time series visual

I think you need to tweak the Top_Cities variable to use ALLSELECTED('Date')

Top Orders | Cities =
VAR N = 'Top N'[Top N Value] -- Get the top N cities by total orders
VAR Top_Cities =
    CALCULATETABLE (
        TOPN ( N, ALLSELECTED ( 'Cities'[City] ), [Total Orders] ),
        ALLSELECTED ( 'Date' )
    ) -- Calculate the total orders for the Top N cities
VAR TopN_Orders =
    CALCULATE ( [Total Orders], KEEPFILTERS ( Top_Cities ) ) -- Calculate total orders for all selected cities
VAR All_Orders =
    CALCULATE ( [Total Orders], ALLSELECTED ( Cities ) ) -- Calculate the total orders for cities that are not in the Top N
VAR Other_Orders =
    All_Orders - CALCULATE ( [Total Orders], Top_Cities )
VAR Result =
    SWITCH (
        TRUE (),
        NOT ISINSCOPE ( Cities[City] ), All_Orders,
        SELECTEDVALUE ( Cities[City] ) = "Others", Other_Orders,
        TopN_Orders
    )
RETURN
    Result