function mvnic(w,f,index)
%MVNIC  Plot Multi-Variable Nichols chart.
%       MVNIC(W,F,INDEX) plots the elements of the MVFR
%       matrix, F, specified by INDEX on a Nichols chart
%       and marks the plots with the frequency points.
%       `Unwrap' is applied to the phases before plotting.
%
%       INDEX is an optional argument which is a two
%       column matrix whose rows specify the elements
%       of F to be plotted.
%       If INDEX is missing all the elements are plotted.
%
%       See also MKNIC,MVBODE,MVNYQ,PLOTNIC,UNWRAP.

%       Dr M.P. Ford 4th August 1987
% Copyright (c) 1987,1993 by GEC Engineering Research Centre and 
% Cambridge Control Ltd. All Rights Reserved.
% History:
%   `fixphase' replaced by `unwrap', 29.5.93, JMM.

if nargin == 3
   f=fget(w,f,index);
else
   f=fget(w,f);
end
plotnic(f)
mknic(f,w)