Unfortunately, there is no member that reflects the current number of undo items saved by the graph. I have created a task to expose the current undo count as a read-only property.
You could get a rough approximation of available undo operations by monitoring axis change events, but this will have limited accuracy, due to the way those events are processed by the graph (for example, the graph combines a continuous mouse drag, composed of many small pans, into a single "undo pan" item).
~ Paul H
