I think I understand your question. Your subvi has the waveform wired as an output. In the main vi that calls the subvi, you wire the output to a graph - correct? So you're looking for the graph properties in the subvi to be transfered to the main vi graph - correct?
If this is correct - it doesn't work that way. The waveform is just a cluster of t0, deltat, and the y array of data. None of the graphs properties are passed out of the subvi.
There are many ways to do what you're trying to do. You could wire the returned strings that you use for x and y labels as outputs of the subvi, then update the property node of the main vi graph. You could also pass a reference to the main vi graph to the subvi and make the x and y label changes to the main vi graph in
the subvi. I think the first option is probably easier.
Tim