FIX: make sure we have more than 1 tick with small log ranges

If we have both a small target number of ticks and a small displayed
range we would previously pick a tick stride that was the same size or
bigger than the visible data range which would result in only 1 tick
being visible on the axis.

This patch ensures that, with a floor at 1, the stride is smaller than
the estimated data range.

re-closes matplotlib#8768  (same symptoms, different cause)

Author: tacaswell

lib/matplotlib/tests/test_ticker.py

@@ -1362,3 +1362,13 @@ def test_bad_locator_subs(sub):
     ll = mticker.LogLocator()
     with pytest.raises(ValueError):
         ll.subs(sub)
+
+
+@pytest.mark.parametrize('numticks', [1, 2, 3, 9])
+@pytest.mark.style('default')
+def test_small_range_loglocator(numticks):
+    ll = mticker.LogLocator()
+    ll.set_params(numticks=numticks)
+    for top in [5, 7, 9, 11, 15, 50, 100, 1000]:
+        ticks = ll.tick_values(.5, top)
+        assert (np.diff(np.log10(ll.tick_values(6, 150))) == 1).all()

lib/matplotlib/ticker.py

@@ -2479,6 +2479,13 @@ def tick_values(self, vmin, vmax):
                   if mpl.rcParams['_internal.classic_mode'] else
                   (numdec + 1) // numticks + 1)
 
+        # if we have decided that the stride is as big or bigger than
+        # the range, clip the stride back to the available range - 1
+        # with a floor of 1.  This prevents getting axis with only 1 tick
+        # visible.
+        if stride >= numdec:
+            stride = max(1, numdec - 1)
+
         # Does subs include anything other than 1?  Essentially a hack to know
         # whether we're a major or a minor locator.
         have_subs = len(subs) > 1 or (len(subs) == 1 and subs[0] != 1.0)