Updated pvtol-nested example to fix matplotlib warnings

The matplotlib hold commands are deprecated, so they have been removed.
Also updated the code for augmenting bode plots to use the new axes labels
that are part of the matplotlib 2.1 modifications.

examples/pvtol-nested.py

@@ -81,26 +81,45 @@
 T = feedback(L, 1);
 
 # Compute stability margins
-#! Not yet implemented
-# (gm, pm, wgc, wpc) = margin(L); 
+(gm, pm, wgc, wpc) = margin(L);
+print("Gain margin: %g at %g" % (gm, wgc))
+print("Phase margin: %g at %g" % (pm, wpc))
 
-#! TODO: this figure has something wrong; axis limits mismatch
 figure(6); clf; 
-bode(L);
+bode(L, logspace(-4, 3));
 
-# Add crossover line
-subplot(211); hold(True);
-loglog([1e-4, 1e3], [1, 1], 'k-')
+# Add crossover line to the magnitude plot
+#
+# Note: in matplotlib before v2.1, the following code worked:
+#
+#   subplot(211); hold(True);
+#   loglog([1e-4, 1e3], [1, 1], 'k-')
+#
+# In later versions of matplotlib the call to subplot will clear the
+# axes and so we have to extract the axes that we want to use by hand.
+# In addition, hold() is deprecated so we no longer require it.
+#
+for ax in gcf().axes:
+    if ax.get_label() == 'control-bode-magnitude':
+        break
+ax.semilogx([1e-4, 1e3], 20 * np.log10([1, 1]), 'k-')
 
+#
 # Replot phase starting at -90 degrees
-bode(L, logspace(-4, 3));
+#
+# Get the phase plot axes
+for ax in gcf().axes:
+    if ax.get_label() == 'control-bode-phase':
+        break
+
+# Recreate the frequency response and shift the phase
 (mag, phase, w) = freqresp(L, logspace(-4, 3));
 phase = phase - 360;
-subplot(212);
-semilogx([1e-4, 1e3], [-180, -180], 'k-')
-hold(True);
-semilogx(w, np.squeeze(phase), 'b-')
-axis([1e-4, 1e3, -360, 0]);
+
+# Replot the phase by hand
+ax.semilogx([1e-4, 1e3], [-180, -180], 'k-')
+ax.semilogx(w, np.squeeze(phase), 'b-')
+ax.axis([1e-4, 1e3, -360, 0]);
 xlabel('Frequency [deg]'); ylabel('Phase [deg]');
 # set(gca, 'YTick', [-360, -270, -180, -90, 0]);
 # set(gca, 'XTick', [10^-4, 10^-2, 1, 100]);
@@ -109,7 +128,6 @@
 # Nyquist plot for complete design
 #
 figure(7); clf;
-axis([-700, 5300, -3000, 3000]); hold(True);
 nyquist(L, (0.0001, 1000));
 axis([-700, 5300, -3000, 3000]);
 
@@ -118,7 +136,6 @@
 
 # Expanded region  
 figure(8); clf; subplot(231); 
-axis([-10, 5, -20, 20]); hold(True);
 nyquist(L);
 axis([-10, 5, -20, 20]);
 
@@ -136,7 +153,7 @@
 
 figure(9); 
 (Yvec, Tvec) = step(T, linspace(0, 20));
-plot(Tvec.T, Yvec.T); hold(True);
+plot(Tvec.T, Yvec.T); 
 
 (Yvec, Tvec) = step(Co*S, linspace(0, 20));
 plot(Tvec.T, Yvec.T);