Ever wondered why you could select gazillion performance counters in real-time, but only 1 or 2 counters when selecting Per Day/Per Week/Per Month/Per Year/Custom? Here is the solution: in the VIClient, go to VirtualCenter Management Configuration and then statistics. By default, the Statistics Collection Level is set to 1 (Basic Metrics).
Ideally, you could set the level to 3 or even 4. After a while, you will notice new performance counters available for selection in the chart options menu! These are great for doing memory sizing/ CPU sizing over a longer period of time. Great huh?