Two possible option:
First, the scales could be messed up such that the data isn't on the screen.
Second, check the data that's being sent to the graph. If there is an error in the calculations you could be generating an array of NaN (Not a Number)values.
Third, there could ba an error being generated somewhere that you aren't seeing. Make sure all the error clusters are chained together and that the end result is appearing on a front panel somewhere.
Mike...