matplotlib · dstansby · Oct 31, 2018 · Oct 10, 2018 · Oct 15, 2018 · Oct 18, 2018
diff --git a/lib/matplotlib/axis.py b/lib/matplotlib/axis.py
@@ -1579,10 +1579,12 @@ def set_major_locator(self, locator):
         locator : ~matplotlib.ticker.Locator
         """
         if not isinstance(locator, mticker.Locator):
-            raise TypeError("formatter argument should be instance of "
+            raise TypeError("locator argument should be instance of "
                             "matplotlib.ticker.Locator")
         self.isDefault_majloc = False
         self.major.locator = locator
+        if self.major.formatter:
+            self.major.formatter._set_locator(locator)
         locator.set_axis(self)
         self.stale = True
 
@@ -1595,10 +1597,12 @@ def set_minor_locator(self, locator):
         locator : ~matplotlib.ticker.Locator
         """
         if not isinstance(locator, mticker.Locator):
-            raise TypeError("formatter argument should be instance of "
+            raise TypeError("locator argument should be instance of "
                             "matplotlib.ticker.Locator")
         self.isDefault_minloc = False
         self.minor.locator = locator
+        if self.minor.formatter:
+            self.minor.formatter._set_locator(locator)
         locator.set_axis(self)
         self.stale = True
 

diff --git a/lib/matplotlib/dates.py b/lib/matplotlib/dates.py
@@ -820,8 +820,14 @@ def __init__(self, locator, tz=None, defaultfmt='%Y-%m-%d'):
                        1. / (MUSECONDS_PER_DAY):
                            rcParams['date.autoformatter.microsecond']}
 
+    def _set_locator(self, locator):
+        self._locator = locator
+
     def __call__(self, x, pos=None):
-        locator_unit_scale = float(self._locator._get_unit())
+        try:
+            locator_unit_scale = float(self._locator._get_unit())
+        except AttributeError:
+            locator_unit_scale = 1
         # Pick the first scale which is greater than the locator unit.
         fmt = next((fmt for scale, fmt in sorted(self.scaled.items())
                     if scale >= locator_unit_scale),

diff --git a/lib/matplotlib/tests/test_dates.py b/lib/matplotlib/tests/test_dates.py
@@ -11,6 +11,7 @@
 import matplotlib.pyplot as plt
 from matplotlib.cbook import MatplotlibDeprecationWarning
 import matplotlib.dates as mdates
+import matplotlib.ticker as mticker
 
 
 def __has_pytz():
@@ -220,6 +221,38 @@ def test_DateFormatter():
     fig.autofmt_xdate()
 
 
+def test_locator_set_formatter():
+    """
+    Test if setting the locator only will update the AutoDateFormatter to use
+    the new locator.
+    """
+    plt.rcParams["date.autoformatter.minute"] = "%d %H:%M"
+    t = [datetime.datetime(2018, 9, 30, 8, 0),
+         datetime.datetime(2018, 9, 30, 8, 59),
+         datetime.datetime(2018, 9, 30, 10, 30)]
+    x = [2, 3, 1]
+
+    fig, ax = plt.subplots()
+    ax.plot(t, x)
+    ax.xaxis.set_major_locator(mdates.MinuteLocator((0, 30)))
+    fig.canvas.draw()
+    ticklabels = [tl.get_text() for tl in ax.get_xticklabels()]
+    expected = ['30 08:00', '30 08:30', '30 09:00',
+                '30 09:30', '30 10:00', '30 10:30']
+    assert ticklabels == expected
+
+    ax.xaxis.set_major_locator(mticker.NullLocator())
+    ax.xaxis.set_minor_locator(mdates.MinuteLocator((5, 55)))
+    decoy_loc = mdates.MinuteLocator((12, 27))
+    ax.xaxis.set_minor_formatter(mdates.AutoDateFormatter(decoy_loc))
+
+    ax.xaxis.set_minor_locator(mdates.MinuteLocator((15, 45)))
+    fig.canvas.draw()
+    ticklabels = [tl.get_text() for tl in ax.get_xticklabels(which="minor")]
+    expected = ['30 08:15', '30 08:45', '30 09:15', '30 09:45', '30 10:15']
+    assert ticklabels == expected
+
+
 def test_date_formatter_strftime():
     """
     Tests that DateFormatter matches datetime.strftime,

diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
@@ -301,6 +301,10 @@ def fix_minus(self, s):
         """
         return s
 
+    def _set_locator(self, locator):
+        """ Subclasses may want to override this to set a locator. """
+        pass
+
 
 class IndexFormatter(Formatter):
     """