This could be caused by lots of reasons, and the best way to diagnose it would be for you to write and include here a small sample that reproduces the problem. Having said that, from your description, the first thing I'd check would be to make sure that the data in the graph is being plotted with the ATTR_DATA_MODE attribute set to VAL_RETAIN and not VAL_DISCARD. If that's not it, then it needs to be investigated further...
Luis Gomes
NI