How to determine the resonant peak magnitude of the closed loop via the Nichols chart?

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I have been studying the control theory and I have recently found the Nichols chart theme. Among others this chart is useful for determining the resonant-peak magnitude of the closed loop based on the open loop frequency response. In my text book there is written on page 446 that the resonant-peak magnitude can be determined as the magnitude of the highest closed-loop contour which is tangent to the curve forming the open loop frequency response. I have been trying to prove validity of this statement for some time but without success. Can anybody help with proving this statement?