Nichols plot
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A Nichols plot is a plot used in signal processing in which the logarithm of the magnitude is plotted against the phase of a frequency response on orthogonal axes. This plot combines the two types of Bode plot — magnitude and phase — on a single graph, with frequency as a parameter along the curve.
This plot is also used in closed loop control systems which have negative feedback. In these systems the plot is useful for assessing the absolute and relative stability of the system. This application of the Nichols plot is central to the Quantitative feedback theory (QFT) of Horowitz and Sidi, a well known method for robust control system design.