WARNING: This page is about a different car, the 2007 Pontiac G5 Pursuit, 2007 Pontiac G5, and 2007 Chevrolet Cobalt. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: View Of Vertical Body Zero Line
Courtesy of GENERAL MOTORS CORP.
The vertical body zero line is an imaginary line drawn through the frame either at the forward or rearward torque box area of the frame.