  Scaled relative graphs were recently introduced to analyze the convergence of
optimization algorithms using two dimensional Euclidean geometry. In this
paper, we connect scaled relative graphs to the classical theory of
input/output systems. It is shown that the Nyquist diagram of an LTI system on
$L_2$ is the convex hull of its scaled relative graph under a particular change
of coordinates. The SRG may be used to visualize approximations of static
nonlinearities such as the describing function and quadratic constraints,
allowing system properties to be verified or disproved. Interconnections of
systems correspond to graphical manipulations of their SRGs. This is used to
provide a simple, graphical proof of the classical incremental passivity
theorem.
