Bug in AutoDateLocator when dates are in reverse order

lib/matplotlib/dates.py

@@ -898,6 +898,10 @@ def get_locator(self, dmin, dmax):
         'Pick the best locator based on a distance.'
         delta = relativedelta(dmax, dmin)
 
+        # take absolute difference
+        if dmin > dmax:
+            delta = -delta
+
         numYears = (delta.years * 1.0)
         numMonths = (numYears * 12.0) + delta.months
         numDays = (numMonths * 31.0) + delta.days

lib/matplotlib/tests/test_coding_standards.py

@@ -96,7 +96,6 @@
                       '*/matplotlib/tri/triinterpolate.py',
                       '*/matplotlib/tests/test_axes.py',
                       '*/matplotlib/tests/test_bbox_tight.py',
-                      '*/matplotlib/tests/test_dates.py',
                       '*/matplotlib/tests/test_delaunay.py',
                       '*/matplotlib/tests/test_dviread.py',
                       '*/matplotlib/tests/test_image.py',

lib/matplotlib/tests/test_dates.py

@@ -162,9 +162,9 @@ def test_DateFormatter():
 def test_date_formatter_callable():
     scale = -11
     locator = mock.Mock(_get_unit=mock.Mock(return_value=scale))
-    callable_formatting_function = lambda dates, _: \
-                        [dt.strftime('%d-%m//%Y') for dt in dates]
-    
+    callable_formatting_function = (lambda dates, _:
+                                    [dt.strftime('%d-%m//%Y') for dt in dates])
+
     formatter = mdates.AutoDateFormatter(locator)
     formatter.scaled[-10] = callable_formatting_function
     assert_equal(formatter([datetime.datetime(2014, 12, 25)]),
@@ -223,7 +223,8 @@ def test_auto_date_locator():
     def _create_auto_date_locator(date1, date2):
         locator = mdates.AutoDateLocator()
         locator.create_dummy_axis()
-        locator.set_view_interval(mdates.date2num(date1), mdates.date2num(date2))
+        locator.set_view_interval(mdates.date2num(date1),
+                                  mdates.date2num(date2))
         return locator
 
     d1 = datetime.datetime(1990, 1, 1)
@@ -275,8 +276,10 @@ def _create_auto_date_locator(date1, date2):
                  '1990-01-01 00:00:40+00:00']
                 ],
                [datetime.timedelta(microseconds=1500),
-                ['1989-12-31 23:59:59.999507+00:00', '1990-01-01 00:00:00+00:00',
-                 '1990-01-01 00:00:00.000502+00:00', '1990-01-01 00:00:00.001005+00:00',
+                ['1989-12-31 23:59:59.999507+00:00',
+                 '1990-01-01 00:00:00+00:00',
+                 '1990-01-01 00:00:00.000502+00:00',
+                 '1990-01-01 00:00:00.001005+00:00',
                  '1990-01-01 00:00:00.001508+00:00']
                 ],
                )
@@ -288,6 +291,21 @@ def _create_auto_date_locator(date1, date2):
                      expected)
 
 
+@image_comparison(baseline_images=['date_inverted_limit'],
+                  extensions=['png'])
+def test_date_inverted_limit():
+    # test ax hline with date inputs
+    t0 = datetime.datetime(2009, 1, 20)
+    tf = datetime.datetime(2009, 1, 31)
+    fig = plt.figure()
+    ax = fig.add_subplot(1, 1, 1)
+    ax.axhline(t0, color="blue", lw=3)
+    ax.set_ylim(t0 - datetime.timedelta(days=5),
+                tf + datetime.timedelta(days=5))
+    ax.invert_yaxis()
+    fig.subplots_adjust(left=0.25)
+
+
 if __name__ == '__main__':
     import nose
     nose.runmodule(argv=['-s', '--with-doctest'], exit=False)