HP 54710A User's Reference Manual page 369

Mask Menu
Edit Mask
Moving an X or Y value off of the graticule area (value is either minimum or
maximum), fixes that point to the edge of the screen. Changing the scaling
will not shrink that point unto the screen. For example, the top line of the
polygon in figure 23-3 looks like both points are off the screen. However,
rescaling the polygon by changing the 1 level (Y2) value from 3 V to 2 V
shrinks the polygon in figure 23-4. You may notice that the top-left point
stayed fixed to the top of the screen, while the top-right point moved into the
graticule area. To keep a point fixed to a side of the graticule area, set the
point's value to either minimum or maximum.
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