Hi,
You're right, it is less general than what I had in mind.
I'll explain how but mind you the Tracking Gantt view has exactly what you
are looking for so if my explanations wouldn't be enough, you can always
have a look on the Bar styles of the Tracking Gantt View
You have the milestone losenge haven't you? It represents current plan
To add a symbol representing the milestone baseline date:
In Bar Styles add a line
Name : Baseline Milestone (or whatever)
Appearance: select a start shape and a color
Show For... Milestone
Row 1
From Baseline Start
To BaselineStart
Hope this helps