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+## Copyright (C) 1998 Auburn University.  All rights reserved.
+##
+## This file is part of Octave.
+##
+## Octave is free software; you can redistribute it and/or modify it
+## under the terms of the GNU General Public License as published by the
+## Free Software Foundation; either version 2, or (at your option) any
+## later version.
+##
+## Octave is distributed in the hope that it will be useful, but WITHOUT
+## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+## FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+## for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with Octave; see the file COPYING.  If not, write to the Free
+## Software Foundation, 59 Temple Place, Suite 330, Boston, MA 02111 USA.
+
+## [mag,phase,w] = nichols(sys[,w,outputs,inputs])
+## Produce Nichols plot of a system
+##
+## Compute the frequency response of a system.
+## inputs:
+##   sys: system data structure (must be either purely continuous or discrete;
+##       see is_digital)
+##   w: frequency values for evaluation.
+##      if sys is continuous, then nichols evaluates G(jw)
+##      if sys is discrete, then nichols evaluates G(exp(jwT)), where T=sys.tsam
+##         (the system sampling time)
+##      default: the default frequency range is selected as follows: (These
+##        steps are NOT performed if w is specified)
+##          (1) via routine bodquist, isolate all poles and zeros away from
+##              w=0 (jw=0 or exp(jwT)=1) and select the frequency
+##             range based on the breakpoint locations of the frequencies.
+##          (2) if sys is discrete time, the frequency range is limited
+##              to jwT in [0,2p*pi]
+##          (3) A "smoothing" routine is used to ensure that the plot phase does
+##              not change excessively from point to point and that singular
+##              points (e.g., crossovers from +/- 180) are accurately shown.
+##   outputs, inputs: the indices of the output(s) and input(s) to be used in
+##     the frequency response; see sysprune.
+## outputs:
+##    mag, phase: the magnitude and phase of the frequency response
+##       G(jw) or G(exp(jwT)) at the selected frequency values.
+##    w: the vector of frequency values used
+## If no output arguments are given, nichols plots the results to the screen.
+## Descriptive labels are automatically placed.  See xlabel, ylable, title,
+## and replot.
+##
+## Note: if the requested plot is for an MIMO system, mag is set to
+## ||G(jw)|| or ||G(exp(jwT))|| and phase information is not computed.
+
+function [mag, phase, w] = nichols (sys, w, outputs, inputs)
+
+  ## check number of input arguments given
+  if (nargin < 1 | nargin > 4)
+    usage("[mag,phase,w] = nichols(sys[,w,outputs,inputs])");
+  endif
+  if(nargin < 2)
+    w = [];
+  endif
+  if(nargin < 3)
+    outputs = [];
+  endif
+  if(nargin < 4)
+    inputs = [];
+  endif
+
+  [f, w] = bodquist(sys,w,outputs,inputs,"nichols");
+
+  [stname,inname,outname] = sysgetsignals(sys);
+  systsam = sysgettsam(sys);
+
+  ## Get the magnitude and phase of f.
+  mag = abs(f);
+  phase = arg(f)*180.0/pi;
+
+  if (nargout < 1),
+    ## Plot the information
+    if(gnuplot_has_multiplot)
+      oneplot();
+    endif
+    gset autoscale;
+    if(gnuplot_has_multiplot)
+      gset nokey;
+    endif
+    clearplot();
+    grid("on");
+    gset data style lines;
+    if(is_digital(sys))
+      tistr = "(exp(jwT)) ";
+    else
+      tistr = "(jw)";
+    endif
+    xlabel("Phase (deg)");
+    if(is_siso(sys))
+      title(["Nichols plot of |[Y/U]",tistr,"|, u=", ...
+        sysgetsignals(sys,"in",1,1), ", y=",sysgetsignals(sys,"out",1,1)]);
+    else
+      title([ "||Y(", tistr, ")/U(", tistr, ")||"]);
+      printf("MIMO plot from\n%s\nto\n%s\n",outlist(inname,"    "), ...
+        outlist(outname,"       "));
+    endif
+    if(max(mag) > 0)
+      ylabel("Gain in dB");
+      md = 20*log10(mag);
+    else
+      ylabel("Gain |Y/U|")
+      md = mag;
+    endif
+
+    axvec = axis2dlim([vec(phase),vec(md)]);
+    axis(axvec);
+    plot(phase,md);
+    mag = phase = w = [];
+  endif
+endfunction