, with the contents of four lines changed to

x1= nx-50  ; Leftmost  grid point to depict, for dimension  x  or  lon
x2= nx     ; Rightmost grid point to depict
y1= ny-40  ; Lowermost grid point to depict, for dimension  y  or  lat
y2= ny     ; Uppermost grid point to depict

in order to zoom in on a sub-domain.

Example 3:
As example 2, but with a modified specification for displaying vectors

The figure below was made by issuing the same command as in Example 1 and 2, after the file userdef.ncl was copied from