Check that stem input is 1D

This also has the side-effect of casting torch Tensors to NumPy arrays,
which fixes matplotlib#30216. Since `stem` is made up of `plot` and `[hv]lines`
whic already do this cast, this just moves it up one level which
prevents doing it twice.

Author: QuLogic

lib/matplotlib/axes/_axes.py

@@ -3436,6 +3436,9 @@ 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

@@ -4744,6 +4744,11 @@ 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."""