BUG: Fix IndexLocator.tick_values returning values greater than vmax

saakshigupta2002 · saakshigupta2002 · commit 54e673c417e1 · 2026-02-06T05:36:46.000+10:30
Fix IndexLocator.tick_values() to not return tick values that exceed vmax. Previously, when using IndexLocator with certain offset values, the generated ticks could exceed the specified vmax, causing issues like colorbar.get_ticks() returning incorrect values for discrete colormaps with NoNorm normalization. The fix adds a filter to remove any tick values greater than vmax after generating the initial tick positions. Fixes #31086
diff --git a/lib/matplotlib/tests/test_ticker.py b/lib/matplotlib/tests/test_ticker.py
@@ -604,6 +604,29 @@ def test_set_params(self):
         assert index._base == 7
         assert index.offset == 7
 
+    def test_tick_values_not_exceeding_vmax(self):
+        """
+        Test that tick_values does not return values greater than vmax.
+        Regression test for issue #31086.
+        """
+        # Test case where offset=0 could cause vmax to be included incorrectly
+        index = mticker.IndexLocator(base=1, offset=0)
+        ticks = index.tick_values(0, 4)
+        assert all(t <= 4 for t in ticks)
+        np.testing.assert_array_equal(ticks, [0, 1, 2, 3, 4])
+
+        # Test case with fractional offset
+        index = mticker.IndexLocator(base=1, offset=0.5)
+        ticks = index.tick_values(0, 4)
+        assert all(t <= 4 for t in ticks)
+        np.testing.assert_array_equal(ticks, [0.5, 1.5, 2.5, 3.5])
+
+        # Test case with base > 1
+        index = mticker.IndexLocator(base=2, offset=0)
+        ticks = index.tick_values(0, 5)
+        assert all(t <= 5 for t in ticks)
+        np.testing.assert_array_equal(ticks, [0, 2, 4])
+
 
 class TestSymmetricalLogLocator:
     def test_set_params(self):
diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
@@ -1767,8 +1767,10 @@ def __call__(self):
         return self.tick_values(dmin, dmax)
 
     def tick_values(self, vmin, vmax):
-        return self.raise_if_exceeds(
-            np.arange(vmin + self.offset, vmax + 1, self._base))
+        ticks = np.arange(vmin + self.offset, vmax + 1, self._base)
+        # Filter out ticks that exceed vmax to avoid off-by-one errors
+        ticks = ticks[ticks <= vmax]
+        return self.raise_if_exceeds(ticks)
 
 
 class FixedLocator(Locator):
