Documentation 
Graphical worstcase gain analysis
wcgainplot(sys)
wcgainplot(sys,w)
wcgainplot(sys,...,options)
wcgainplot(sys) plots the nominal and worstcase gains of the uncertain system sys as a function of frequency. For multiinput, multioutput (MIMO) systems, gain refers to the largest singular value of the frequency response matrix. (See sigma for more information about singular values.) The plot includes:
Nominal — nominal gain of sys
Worst — the response falling within the uncertainty of sys that has the highest peak gain
Worstcase gain (lower bound) — the lowest worstcase gain at each frequency
Worstcase gain (upper bound) — the highest gain within the uncertainty at each frequency
Sampled Uncertainty — 20 responses randomly sampled from sys
wcgainplot(sys,w) focuses the plot on the frequencies specified by w.
If w is a cell array {wmin,wmax}, wcgainplot plots the worstcase gains in the range {wmin,wmax}.
If w is an array of frequencies, wcgainplot plots the worstcase gains at each frequency in the array.
wcgainplot(sys,...,options) uses the options set options to specify additional options for the computation of the worstcase gains. Use wcgainOptions to create the options set.
sys 
Uncertain dynamic system. 
w 
Frequencies of worstcase gain plots. Specify frequencies in radians/TimeUnit, where TimeUnit is the time unit of sys.

options 
Options set specifying additional options for the computation of the worstcase gains. Use wcgainOptions to create the options set. 
sigma  usample  wcgain  wcgainOptions