The SQL Server chart "Disk Queue Length" is displaying data from disk/drives which are not used by the selected SQL Server instance.
The selected SQL Server instance is using drive 'G:' for its data and log files. But in the chart also data from drive 'F:' is displayed. Drive 'F:' is used by another SQL Server instance.
Clustered SQL Server environment
In SQL Server cartridges previous to 5.5.8, Foglight picks up the information about the disks/drives from the node, but in the cluster environment the information needs to be picked up from the cluster administration.