The behaviour of the -scale option is actually documented:
"scale scale_data
New scale data for the axis, i.e. a 3-element list containing minimum, maximum and stepsize for the axis, in this order.
Beware: Setting this option will clear all data from the plot."
I checked the code (did not try a small test though) and it is supposed to make sure the clearing only occurs if you reset the scale. But I should probably test it
Regards,
Arjen