When time range is configured for last N minutes PXC/Galera Graphs is really unstable – after each refresh, it returns (slightly) different result set.
How to repeat:
1. Open https://pmmdemo.percona.com/graph/d/lEoftfWiz/pxc-galera-graphs?orgId=1&from=now-30m&to=now&panelId=42&fullscreen&var-interval=$__auto_interval_interval&var-instance=All&var-cluster=pxc56-cluster&var-host=pxc56-2&refresh=1m
2. Click the refresh button a few times.
Graphs should be more consistent after refresh.