Could it be that the waterfall graph has a lower speed mode setting than the other graph? By default if speed mode is used, a watermark shows on graph, but that could have been globally turned off using the Tools->Options->Graph tab check box.
Bring up Plot Details dialog on the waterfall graph, go to Layer level on left panel, then on right panel select Size/Speed tab and check the speed mode number of points setting, and try increasing to a larger number if that number is too small.