[gradsusr] Is any command to get line plot's max-min value?

[清风]

Hi,everyone
 Recently,i  try to write a new axis style,when it comes to 'set gxout line',i need to know the max and min value of the 'line',but it seems that there is no such command.
 Can anyone please give me some suggestion？Thanks a lot.
 The attach shows my new axis when set gxout shaded with z and y varying.
 best regards.
  
 QingFeng from China
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