Seems like this is supposed to be resolved with the recent release of vCenter 5.1 U1b:

https://www.vmware.com/support/vsphere5/doc/vsphere-vcenter-server-51u1b-release-notes.html

vSphere Web Client might fail to display the advanced performance charts

When you use a German locale vSphere Web Client to view advanced performance charts, an error message similar to the following might be displayed:

Performance data is not currently available for this entry

In addition, a notification message similar to the following might also be displayed:

For input string <x>.<x>

This issue is resolved in this release.

I haven't testet this myself yet, can anyone confirm this resolves it?

I'm on a German client but and I've set my browser to send English Accept-Language headers, so I'm actually browsing the English version Web Client already though.