Disk Usage Line Charge

SteveA​ In the Line Chart visualization screenshot you have the widget configured for expecting context from another widget with a context of type "File System". Not only is consuming from context optional for this widget, such a context type is only set by another File System widget. As you described aiming to "view all disks on a windows server", if intending to drive that by context rather than a static device selection via filter, you may be intending to drive that via device selection within another widget in which case you will need to change the Type field to "Device" followed by selecting your expected device context in Which Set of Selections.

The Forecast visualization the widget configuration depicted was configured for consuming context of type "Device" which is likely why you were immediately getting preview data as you already have a device widget setting context within your dashboard.