OOOOhhh, yeah I've never used that feature but it seems to be a feature of the Waveform Chart and not the graph.
http://forums.ni.com/t5/LabVIEW/How-to-stack-plots-using-Waveform-Graph/td-p/329917
The good news is a Mixed Signal Graph can look and behave in a similar way.
Graph Test 2.vi