Backport PR matplotlib#30233: Check that stem input is 1D

Author: dstansby

lib/matplotlib/axes/_axes.py

@@ -3171,6 +3171,9 @@ def stem(self, *args, linefmt=None, markerfmt=None, basefmt=None, bottom=0,
         else:  # horizontal
             heads, locs = self._process_unit_info([("x", heads), ("y", locs)])
 
+        heads = cbook._check_1d(heads)
+        locs = cbook._check_1d(locs)
+
         # resolve line format
         if linefmt is None:
             linefmt = args[0] if len(args) > 0 else "C0-"

lib/matplotlib/tests/test_axes.py

@@ -4538,6 +4538,11 @@ def _assert_equal(stem_container, expected):
     _assert_equal(ax.stem(y, linefmt='r--'), expected=([0, 1, 2], y))
     _assert_equal(ax.stem(y, 'r--'), expected=([0, 1, 2], y))
 
+    with pytest.raises(ValueError):
+        ax.stem([[y]])
+    with pytest.raises(ValueError):
+        ax.stem([[x]], y)
+
 
 def test_stem_markerfmt():
     """Test that stem(..., markerfmt=...) produces the intended markers."""