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+# Copyright (C) 1998 A. Scottedward Hodel 
+#
+# 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, 675 Mass Ave, Cambridge, MA 02139, USA. 
+ 
+function [mag,phase,w] = nichols(sys,w,outputs,inputs)
+# [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.
+
+# $Log$
+
+  # 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)");
+    ylabel("Gain in dB");
+    if(is_siso(sys))
+      title(["Nichols plot of |[Y/U]",tistr,"|, u=", inname, ...
+	", y=",outname]);
+    else
+      title([ "||Y(", tistr, ")/U(", tistr, ")||"]);
+      disp("MIMO plot from")
+      outlist(inname,"	");
+      disp("to")
+      outlist(outname,"	");
+    endif
+    md = 20*log10(mag);
+    axvec = axis2dlim([vec(phase),vec(md)]);
+    axis(axvec);
+    plot(phase,md);
+    mag = phase = w = [];
+  endif
+endfunction