To prevent the vertical scale from adjusting while you are taking data, you will need to deselect the auto-scale during sampling option. This option appears in the graph setup window.
To access the graph setup window, click the button in the lower lefthand corner of the graph window. Next, click the box to the left of "Auto-Scale During Sampling" to remove the check. Click OK to exit the graph setup window and collect data as usual.
This experiment setup can be saved by selecting Save As from the File menu.