Welcome to Tableau Community!
And I really appreciate your polite manner of asking question.
My approach for your question is color trick.
The main steps are,
 create a flag field for dividing actual and forecast
 use this flag as color mark
 assign white to the lines to make them "invisible"
In this screenshot, I assigned gray for the lines to show their existence.
Change the gray to white.
Another point is automatic calculation of showing actual and forecast.
There are two options.
 Compare with today
After today: show forecast
Today or before: show actual
Please refer to worksheet "Flag by Today", which is using [flag1]
 Compare with data
Find the max date of non-null actual value.
After max date: show forecast
Max date or before: show actual
Please refer to worksheet "Flag by Data", which is using [flag2]
Runningsumquestion.twbx 170.4 KB
To overlay the color on continuous line chart is little bit tough.
Hope this helps.
Runningsumquestion_SM_10.5.twbx 151.8 KB
Thanks very much, that works perfectly!
I have a follow-up question if you wouldn't mind. I also want to add some text boxes to my dashboard using the figures from that line graph.
So for example:
Closing Balance at Latest Business Date: 1,620,829
I think I've achieved this with the calculation:
IF [Date] = [Latest Business Date] then [Actual Close] END
But I want to do one for:
Forecast Closing Balance at close of next Month: 1,695,917
but the calculation: IF ATTR([Date]) = [Last Day of Month] then [Actual + FCST] END
Doesn't work. Is this because [Actual + FCST] is a table calculation?
Any help would be appreciated.
Thank you very much that was very helpful. The non-null actual value trick especially is a big improvement for my dashboard.