Fix more edge cases in psd, csd.

intervaly would not be ordered if the yaxis is inverted; get_ybound() is
always ordered.  Fix that, add a test, and also test the full-zero input
case recently fixed.

Author: anntzer

lib/matplotlib/axes/_axes.py

@@ -7042,7 +7042,7 @@ def psd(self, x, NFFT=None, Fs=None, Fc=None, detrend=None,
         self.set_ylabel('Power Spectral Density (%s)' % psd_units)
         self.grid(True)
 
-        vmin, vmax = self.viewLim.intervaly
+        vmin, vmax = self.get_ybound()
         step = max(10 * int(np.log10(vmax - vmin)), 1)
         ticks = np.arange(math.floor(vmin), math.ceil(vmax) + 1, step)
         self.set_yticks(ticks)
@@ -7144,7 +7144,7 @@ def csd(self, x, y, NFFT=None, Fs=None, Fc=None, detrend=None,
         self.set_ylabel('Cross Spectrum Magnitude (dB)')
         self.grid(True)
 
-        vmin, vmax = self.viewLim.intervaly
+        vmin, vmax = self.get_ybound()
         step = max(10 * int(np.log10(vmax - vmin)), 1)
         ticks = np.arange(math.floor(vmin), math.ceil(vmax) + 1, step)
         self.set_yticks(ticks)

lib/matplotlib/tests/test_axes.py

@@ -4671,6 +4671,16 @@ def test_spectrum():
             ax.set(xlabel="", ylabel="")
 
 
+def test_psd_csd_edge_cases():
+    # Inverted yaxis or fully zero inputs used to throw exceptions.
+    axs = plt.figure().subplots(2)
+    for ax in axs:
+        ax.yaxis.set(inverted=True)
+    with np.errstate(divide="ignore"):
+        axs[0].psd(np.zeros(5))
+        axs[1].csd(np.zeros(5), np.zeros(5))
+
+
 @check_figures_equal(extensions=['png'])
 def test_twin_remove(fig_test, fig_ref):
     ax_test = fig_test.add_subplot()