The Monitorable Items list on the ServiceNow Performance dashboard selects which set of data will be displayed, since generally ServiceNow instances outside of PDIs run on several servers at a time to allow for redundancy against downtime.
Information on node names for the given instance has been historically registered as separate records under the sys_cluster_state table records.
Apparently there's also a separate graph to show data from multiple instances at a time. Use the Graph Set list to change to "Instance View". You can see this SN doc page for more details, including other Graph Sets for this dashboard:
https://docs.servicenow.com/bundle/washingtondc-platform-administration/page/administer/platform-per...
You can also use an web browser extension to quickly switch between them, e.g. with the "SN Utils" browser plugin available for Firefox, Chrome, Safari:
https://apps.apple.com/us/app/sn-utils/id1542209967
https://addons.mozilla.org/en-US/firefox/addon/servicenow-utils2/
https://chromewebstore.google.com/detail/sn-utils-tools-for-servic/jgaodbdddndbaijmcljdbglhpdhnjobg
You can view the node name that you're currently signed into through the stats.do page, e.g.:
https://AcmeCompanyDevInstance.service-now.com/stats.do
The name will be listed in the "Connected to cluster node" line.
