Hi CC,
The process is slightly different depending on whether the graph is in VIEW or in REPORT. In VIEW you can simply double-click on the legend and select the "unit_string" property from the list of available channel properties on the left of the dialog.
In REPORT you should double-click on the legend, then select "Free text" as the labeling mode, then type in the expression:
@ChnDim(CCN)@
Brad Turpin
DIAdem Product Support Engineer
National Instruments