Oh right, seems like you've ran into the same problem as I was after upgrading from 4.1 to 5.0. VMware removed the per-VM IOPS counters from the default vCenter statistics collection levels, so your vCenter database does not store any historical data about them anymore.
You can change this behaviour as described in this post:
Unfortunately for you, this means you don't have any access to past data on a per-VM basis. You would need to either start from scratch after making the changes above and wait until you get some amount of samples you're comfortable with, or refer to host-level total counters.
The latter could be quite skewed and inaccurate however, due to non-VM generated IO like StoragevMotions, cloning operations or things like VAAI.