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My company standard uses an incomplete box. If the part (we generally
do computer screens) you want is on the bottom, then you discontinue
the screen above your point of interest by leaving the surrounding box
open at the top.
If the part you want is on the top, then you discontinue the screen
below your point of interest by leaving the surrounding box open at
the bottom.
If the part you want is in the middle, then both the top and bottom of
the box are left off, with only the sides remaining.
This assumes that you surround your graphic with boxes. Hope this
helps!