I'm not sure if this would work, but you might be able to create a calculated field that displays either STR1 or STR2 and use it for the label. For example,
IF [Mobile]="Capacity" then [STR1]
ELSIEF [Mobile]="RefCapacity" then [STR2]
Hope this helps
I might have not been clear but Mobile is a Device type where "Device" has been plotted as a column. "Capacity" and "RefCapacity" are measure values coming from different data sources.