Fix missing parameter initialization in Axes.specgram()

lib/matplotlib/axes/_axes.py

@@ -7617,6 +7617,8 @@ def specgram(self, x, NFFT=None, Fs=None, Fc=None, detrend=None,
             Fc = 0  # same default as in mlab._spectral_helper()
         if noverlap is None:
             noverlap = 128  # same default as in mlab.specgram()
+        if Fs is None:
+            Fs = 2  # same default as in mlab._spectral_helper()
 
         if mode == 'complex':
             raise ValueError('Cannot plot a complex specgram')

lib/matplotlib/tests/test_axes.py

@@ -4166,6 +4166,18 @@ def test_specgram_angle():
                                 scale="dB")
 
 
+def test_specgram_fs_none():
+    """Test axes.specgram when Fs is None, should not throw error."""
+
+    try:
+        spec, freqs, t, im = plt.specgram(np.ones(300), Fs=None)
+    except Exception as e:
+        raise pytest.fail("DID RAISE {0}".format(e))
+
+    xmin, xmax, freq0, freq1 = im.get_extent()
+    assert xmin == 32 and xmax == 96
+
+
 @image_comparison(
     ["psd_freqs.png", "csd_freqs.png", "psd_noise.png", "csd_noise.png"],
     remove_text=True, tol=0.002)