This article provides information on a known issue.
Symptoms:
Disk usage for the 10 virtual machines in the ESXi host performance over chart is not displayed.
Reference document about the metric can be found here
The data is not available only for Virtual Machines on vSAN a datastores. Virtual machines on other datastore should display data correctly in the charts.
vSAN does not support this legacy metric in vCenter performance chart and the data for vSAN is collected via a different API and User Interface.
vSAN 6.2 and above has its own "Performance Service" which has its own set of features, APIs and UIs. The vSAN performance metrics are in separate view and the top contributors in a vSAN datastore can be viewed