No changes in zoomed out Cluster Size Graph if node was absent for a short time

Hi,

I had some node restarts and so we can see cluster size was changing
appropriately:

https://18.220.168.208/graph/dashboard/db/pxc-galera-cluster-overview?orgId=1&from=1521936275384&to=1521939574076

However if I zoom out to about last 24 hours it looks like Cluster did NOT
ave any times it operated in reduced size

https://18.220.168.208/graph/dashboard/db/pxc-galera-cluster-overview?orgId=1&from=1521908120097&to=1521991518319

The problem is we're not using min_over_time to match the selected
$interval as such if the time when node left cluster was short it could not
be picked up with gauge "sampling" which happens.

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