I found this definition and complete description in the documentation. I've also included the definition here.
Percent Memory Used Chart
Provides a chart of the percentage of total memory available used by the selected node over a custom period of time.
If you are looking for the OID for this, post me back and I'll help you find that information.
Hope this helps,
I was wondering if you could give me more information about the specific OID for this calculation. Let me know if there is a better way to reach you then directly on these forums. Not sure if you still have the information or not as it has been awhile since you posted.