Line graph with solid and dotted lines

You can use 2 measures for this. I have taken your sample data and made the below:

My two measures are as follows, starting with Previous Months:

VAR month_now =
    MONTH ( NOW () )
VAR calc =
    CALCULATE ( SUM ( 'Table'[Data 1] ), KEEPFILTERS ( 'Table'[Month Number] < 5 ) )
RETURN
    calc

The next one does current and future months:

VAR month_now =
    MONTH ( NOW () )
VAR calc =
    CALCULATE (
        SUM ( 'Table'[Data 1] ),
        KEEPFILTERS ( 'Table'[Month Number] >= month_now )
    )
RETURN
    calc

Once you have put the two measures in the Y-Axis of the chart, you can set the Line settings for the Current Month Line differently:

If this works, please mark this as the solution.