Yannick,
You are correct in your interpretation of this attribute. There actually is a bug in 7.0 that is causing the graph to needlessly refresh its plots under some conditions. Normally that would be unnoticeable (although it would slow down plotting), but if you use VAL_DISCARD, then you would see this behavior.
If you want to continue using VAL_DISCARD, there is a workaround you can use until the problem is fixed: make sure that your *top* x-axis has autoscaling turned off. You can do this in the UI Editor, or you can do it programmatically, using the following code:
SetAxisScalingMode (panel, control, VAL_TOP_XAXIS, VAL_LOCK, 0, 0);
Sorry about the inconvenience.
Luis Gomes
NI