I ran into this when building an xControl, but I found out that it is generic to clusters, and doesn't seem to have any relation to the xControl. A waveform graph in a cluster will not autoscale its axes when new data is written to it, and it is on a page of a tab control that is not visible. As a work-around, you can force the data into a property node of the waveform graph (in the cluster) before writing data to the cluster.
I've attached a demo vi to demonstrate. I wrote the vi in 8.6 and I've also tested it in 8.5; it works the same in both (i.e., incorrectly).
Is this a bug or a "feature"?
Chris
