Virtualmin displayed wrong amount of available memory one OpenVZ systems for a while (displaying the guaranteed RAM instead of the burst or some random smaller value).
After the update to Virtualmin 4.02, the available RAM graph completely vanished from the system information page.
After searching for information about this small issue (and not finding much), I eventually found a solution: installing an updated Webmin Processes module, available at http://www.webmin.com/updates/proc-1.630-2.wbm.gz
To do this navigate to Webmin > Webmin Configuration > Webmin Modules. Upload the (previously saved) gz file or paste the URL and install the updated Processes module.
After this, navigate back to the system information page and request an update. You will notice the memory graph will return in all its amazing glory.