It would be great if you attach a sample workbook to help you on this.
I'm not sure if this will work for your true setup,
but you could try left-joining to a date-scaffold
which contains all the possible dates you want.
Then you would use the scaffoldDate to plot
as Discrete and should be able to get your refline.
Please see workbook v10.3 and sample datasource attached and in the Forum Thread:
This works beautifully - thanks