Backport PR matplotlib#31706: Fix clabel manual index

Author: rahulrathnavel

lib/matplotlib/contour.py

@@ -478,8 +478,8 @@ def add_label_near(self, x, y, inline=True, inline_spacing=5,
         label_width = self._get_nth_label_width(level)
         rotation, path = self._split_path_and_get_label_rotation(
             path, idx_vtx_min, proj, label_width, inline_spacing)
-        self.add_label(*proj, rotation, self.labelLevelList[idx_level_min],
-                       self.labelCValueList[idx_level_min])
+        self.add_label(*proj, rotation, self.labelLevelList[level],
+                       self.labelCValueList[level])
 
         if inline:
             self._paths[idx_level_min] = path

lib/matplotlib/tests/test_contour.py

@@ -891,3 +891,10 @@ def test_contour_aliases(fig_test, fig_ref):
 def test_contour_singular_color():
     with pytest.raises(TypeError):
         plt.figure().add_subplot().contour([[0, 1], [2, 3]], color="r")
+
+
+def test_clabel_manual_subset():
+    fig, ax = plt.subplots()
+    cs = ax.contour([[1, 2], [3, 4]], levels=[1.5, 2.5, 3.5])
+    # Attempt to label only one specific level manually
+    ax.clabel(cs, levels=[2.5], manual=[(0.5, 0.5)])