Not sure if I understood your requirement correctly. Is your requirement to generate two line graphs for two key figures (Budget & Actual) and Not taking Budgeted Zeroes into consideration while populating the line item chart for Prior Months.
Isn't there any number sitting in Budgeted cells for prior months?
you could create a third line item "Actual & Budget Revenue" that takes "Actual revenue" for the closed months and "Budget revenue" for the open months and create a line chart with "Actual & Budget Revenue" and "Actual revenue" (in this order to have visible the split between closed and open periods in the graph).
You should have a result similar to the example attached.