You can't jump data unit in drill down so.... The way I've gotten around it is to present the Actual scenario (and in your case, cube) for the actual months of the year and the forecast for the remaining months of the forecast scenario in one cube view. The data should be the same in the actual months in both actuals and forecast scenarios so it's still there, but you do kind of run the risk of those two scenarios not being in sync (albeit very low). But if it's Budget where you're manually entering on top of the seeded actuals, you can link a cube view or navigation link that would bring you back over to the actuals cube where the user could drill down
it would be a great solution if you figure out a way to do it with something on the front end. I'm not aware of any way to modify the drill down code (it's back end?) to allow jumping data units.
