ConciseDateFormatter's offset string is correct on an inverted axis (matplotlib#28501)

Author: OdileVidrine

doc/api/next_api_changes/behavior/28501-OV.rst

@@ -0,0 +1,5 @@
+The offset string associated with ConciseDateFormatter will now invert when the axis is inverted
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Previously, when the axis was inverted, the offset string associated with ConciseDateFormatter would not change,
+so the offset string indicated the axis was oriented in the wrong direction. Now, when the axis is inverted, the offset
+string is oriented correctly.

lib/matplotlib/dates.py

@@ -796,7 +796,12 @@ def format_ticks(self, values):
 
         if show_offset:
             # set the offset string:
-            self.offset_string = tickdatetime[-1].strftime(offsetfmts[level])
+            if (self._locator.axis and
+                    self._locator.axis.__name__ in ('xaxis', 'yaxis')
+                    and self._locator.axis.get_inverted()):
+                self.offset_string = tickdatetime[0].strftime(offsetfmts[level])
+            else:
+                self.offset_string = tickdatetime[-1].strftime(offsetfmts[level])
             if self._usetex:
                 self.offset_string = _wrap_in_tex(self.offset_string)
         else:

lib/matplotlib/tests/test_dates.py

@@ -636,6 +636,23 @@ def test_concise_formatter_show_offset(t_delta, expected):
     assert formatter.get_offset() == expected
 
 
+def test_concise_formatter_show_offset_inverted():
+    # Test for github issue #28481
+    d1 = datetime.datetime(1997, 1, 1)
+    d2 = d1 + datetime.timedelta(days=60)
+
+    fig, ax = plt.subplots()
+    locator = mdates.AutoDateLocator()
+    formatter = mdates.ConciseDateFormatter(locator)
+    ax.xaxis.set_major_locator(locator)
+    ax.xaxis.set_major_formatter(formatter)
+    ax.invert_xaxis()
+
+    ax.plot([d1, d2], [0, 0])
+    fig.canvas.draw()
+    assert formatter.get_offset() == '1997-Jan'
+
+
 def test_concise_converter_stays():
     # This test demonstrates problems introduced by gh-23417 (reverted in gh-25278)
     # In particular, downstream libraries like Pandas had their designated converters