Merge pull request matplotlib#21785 from jklymak/fix-concise-offset-zoom

Fix ConciseDateFormatter offset during zoom

Author: QuLogic

lib/matplotlib/dates.py

@@ -791,15 +791,15 @@ def format_ticks(self, values):
         # offset fmt are for the offset in the upper left of the
         # or lower right of the axis.
         offsetfmts = self.offset_formats
+        show_offset = self.show_offset
 
         # determine the level we will label at:
         # mostly 0: years,  1: months,  2: days,
         # 3: hours, 4: minutes, 5: seconds, 6: microseconds
         for level in range(5, -1, -1):
             if len(np.unique(tickdate[:, level])) > 1:
-                # level is less than 2 so a year is already present in the axis
-                if (level < 2):
-                    self.show_offset = False
+                if level < 2:
+                    show_offset = False
                 break
             elif level == 0:
                 # all tickdate are the same, so only micros might be different
@@ -839,11 +839,13 @@ def format_ticks(self, values):
                     if '.' in labels[nn]:
                         labels[nn] = labels[nn][:-trailing_zeros].rstrip('.')
 
-        if self.show_offset:
+        if show_offset:
             # set the offset string:
             self.offset_string = tickdatetime[-1].strftime(offsetfmts[level])
             if self._usetex:
                 self.offset_string = _wrap_in_tex(self.offset_string)
+        else:
+            self.offset_string = ''
 
         if self._usetex:
             return [_wrap_in_tex(l) for l in labels]

lib/matplotlib/tests/test_dates.py

@@ -604,6 +604,28 @@ def test_concise_formatter_show_offset(t_delta, expected):
     assert formatter.get_offset() == expected
 
 
+def test_offset_changes():
+    fig, ax = plt.subplots()
+
+    d1 = datetime.datetime(1997, 1, 1)
+    d2 = d1 + datetime.timedelta(weeks=520)
+
+    locator = mdates.AutoDateLocator()
+    formatter = mdates.ConciseDateFormatter(locator)
+    ax.xaxis.set_major_locator(locator)
+    ax.xaxis.set_major_formatter(formatter)
+
+    ax.plot([d1, d2], [0, 0])
+    fig.draw_without_rendering()
+    assert formatter.get_offset() == ''
+    ax.set_xlim(d1, d1 + datetime.timedelta(weeks=3))
+    fig.draw_without_rendering()
+    assert formatter.get_offset() == '1997-Jan'
+    ax.set_xlim(d1, d1 + datetime.timedelta(weeks=520))
+    fig.draw_without_rendering()
+    assert formatter.get_offset() == ''
+
+
 @pytest.mark.parametrize('t_delta, expected', [
     (datetime.timedelta(weeks=52 * 200),
      ['$\\mathdefault{%d}$' % (t, ) for t in range(1980, 2201, 20)]),