For the Matlab bode diagram, normally when we want to obtain the gain margin, we have to cross the line from phase -180 and touch the curve. (Picture1 is what I did) however from the matlab, the gain margin crossover is start from -540 instead of -180 degree by using "Margin" function(Picture2 is what Matlab shown). Attached is the problem's picture, may I know should I stick with -180 like the "picture1" ? or just follow what the Matlab show which is -540? What i guess is, due to the gain margin is positive, thus the system auto change to -540 so the gain margin will be negative? Does this make any sense?