No it is not. Ask a full scale pilot about their manifold pressure gauge, it reads absolute pressure starting at 0, though some start at a higher pressure because it is impossible to have 0 pressure. The gage sits around 30" sitting on the ground with the engine off. Ask a mechanic about his vacuum gauge, the needle sits at 0 when not used, and can go above 0 and down to no more than 30" of vacuum.