Same thing happening to me but not on all hosts... was hoping there would be an answer to this...
Solved my own issue, here's how I did it (for documentation purposes)
- Restarted Web Management Services on the vCenter Server - this should have done it but that didn't work either, same results.
- Restarted Management Network on the problematic ESXi hosts - another good troubleshooting step I thought but this didn't work either.
- Restarted Management Agents on the problematic ESXi hosts - THIS WORKED and resolved the issue, performance graphs got "Realtime" data back.
Only thing I would recommend before doing this last step is maybe disabling Host Monitoring at the cluster-level, as when the management agents are restarted it will freak vCenter out and think that a host failed. None of my VM's went down and the host stayed up of course, just threw lots of alerts in vCenter.
Hope this helps someone!