Fix very-edge case in csd(), plus small additional cleanups.

Previously, `csd([0, 0], [0, 0])` would fail with a ZeroDivisionError
when computing yticks.  Fix that by copying the logic from psd, and
streamlining it in both places.  Also fix an unformatted exception
message in specgram.

Author: anntzer

lib/matplotlib/axes/_axes.py

@@ -7035,12 +7035,9 @@ def psd(self, x, NFFT=None, Fs=None, Fc=None, detrend=None,
         self.set_xlabel('Frequency')
         self.set_ylabel('Power Spectral Density (%s)' % psd_units)
         self.grid(True)
+
         vmin, vmax = self.viewLim.intervaly
-        intv = vmax - vmin
-        logi = int(np.log10(intv))
-        if logi == 0:
-            logi = .1
-        step = 10 * logi
+        step = max(10 * int(np.log10(vmax - vmin)), 1)
         ticks = np.arange(math.floor(vmin), math.ceil(vmax) + 1, step)
         self.set_yticks(ticks)
 
@@ -7140,11 +7137,9 @@ def csd(self, x, y, NFFT=None, Fs=None, Fc=None, detrend=None,
         self.set_xlabel('Frequency')
         self.set_ylabel('Cross Spectrum Magnitude (dB)')
         self.grid(True)
-        vmin, vmax = self.viewLim.intervaly
-
-        intv = vmax - vmin
-        step = 10 * int(np.log10(intv))
 
+        vmin, vmax = self.viewLim.intervaly
+        step = max(10 * int(np.log10(vmax - vmin)), 1)
         ticks = np.arange(math.floor(vmin), math.ceil(vmax) + 1, step)
         self.set_yticks(ticks)
 
@@ -7576,7 +7571,7 @@ def specgram(self, x, NFFT=None, Fs=None, Fc=None, detrend=None,
             else:
                 Z = 20. * np.log10(spec)
         else:
-            raise ValueError('Unknown scale %s', scale)
+            raise ValueError(f'Unknown scale {scale!r}')
 
         Z = np.flipud(Z)