diff --git a/spatialmath/base/graphics.py b/spatialmath/base/graphics.py
index 2baf94b7..97368801 100644
--- a/spatialmath/base/graphics.py
+++ b/spatialmath/base/graphics.py
@@ -1513,7 +1513,10 @@ def _axes_dimensions(ax: plt.Axes) -> int:
             # handle the case of Animate objects pretending to be Axes
             classname = ax.__class__.__name__
             base_classes = ax.__class__.__bases__
-            if classname in ("Axes3DSubplot", "Animate") or any(base_class.__name__ in ("Axes3DSubplot", "Animate") for base_class in base_classes):
+            if classname in ("Axes3DSubplot", "Animate") or any(
+                base_class.__name__ in ("Axes3DSubplot", "Animate")
+                for base_class in base_classes
+            ):
                 return 3
             elif classname in ("AxesSubplot", "Animate2"):
                 return 2
@@ -1736,20 +1739,24 @@ def plotvol3(
         ax._plotvol = True
         return ax
 
-    def expand_dims(dim: ArrayLike = None, nd: int = 2) -> NDArray:
+    def expand_dims(dim: ArrayLike | None = None, nd: int = 2) -> NDArray | None:
         """
         Expand compact axis dimensions
 
-        :param dim: dimensions, defaults to None
+        :param dim: dimensions, or None if unspecified, defaults to None
         :type dim: scalar, array_like(2), array_like(4), array_like(6), optional
-        :param nd: number of axes dimensions, defaults to 2
+        :param nd: number of axes dimensions (2 or 3), defaults to 2
         :type nd: int, optional
         :raises ValueError: bad arguments
-        :return: 2d or 3d dimensions vector
-        :rtype: ndarray(4) or ndarray(6)
+        :return: 2d or 3d dimensions vector, or None if unspecified
+        :rtype: ndarray(4), ndarray(6), or None
 
         Compute bounding dimensions for plots from shorthand notation.
 
+        ``None`` is returned unchanged so the caller can choose whether to
+        autoscale or obtain dimensions from another source. No default extent
+        is inferred.
+
         If ``nd==2``, [xmin, xmax, ymin, ymax]:
             * A -> [-A, A, -A, A]
             * [A,B] -> [A, B, A, B]
@@ -1766,6 +1773,11 @@ def expand_dims(dim: ArrayLike = None, nd: int = 2) -> NDArray:
         corresponding maximum, matplotlib will invert that axis when the limits
         are applied, which is a convenient way to flip an axis direction.
         """
+        if nd not in (2, 3):
+            raise ValueError("nd is 2 or 3")
+        if dim is None:
+            return None
+
         dim = smb.getvector(dim)
 
         if nd == 2:
@@ -1788,8 +1800,6 @@ def expand_dims(dim: ArrayLike = None, nd: int = 2) -> NDArray:
                 return dim
             else:
                 raise ValueError("bad dimension specified")
-        else:
-            raise ValueError("nd is 2 or 3")
 
     def set_window_title(fig: plt.Figure, title: str) -> bool:
         """
diff --git a/tests/base/test_graphics.py b/tests/base/test_graphics.py
index 17b88735..2d5223c4 100644
--- a/tests/base/test_graphics.py
+++ b/tests/base/test_graphics.py
@@ -46,6 +46,32 @@ class TestGraphics(unittest.TestCase):
     def teardown_method(self, method):
         plt.close("all")
 
+    def test_expand_dims_unspecified(self):
+        self.assertIsNone(expand_dims())
+        for nd in (2, 3):
+            with self.subTest(nd=nd):
+                self.assertIsNone(expand_dims(nd=nd))
+                self.assertIsNone(expand_dims(None, nd=nd))
+
+    def test_expand_dims_specified(self):
+        for nd in (2, 3):
+            with self.subTest(nd=nd):
+                # Zero is an explicit extent, not an unspecified one.
+                np.testing.assert_array_equal(expand_dims(0, nd), [0, 0] * nd)
+                np.testing.assert_array_equal(expand_dims(3, nd), [-3, 3] * nd)
+                np.testing.assert_array_equal(expand_dims([-2, 5], nd), [-2, 5] * nd)
+                limits = np.arange(2 * nd)
+                np.testing.assert_array_equal(expand_dims(limits, nd), limits)
+                with self.assertRaises(ValueError):
+                    expand_dims([1, 2, 3], nd)
+
+    def test_expand_dims_invalid_nd(self):
+        for nd in (1, 4):
+            for dim in (None, 1):
+                with self.subTest(nd=nd, dim=dim):
+                    with self.assertRaisesRegex(ValueError, "nd is 2 or 3"):
+                        expand_dims(dim, nd)
+
     def test_expand_dims(self):
         np.testing.assert_array_equal(expand_dims(3), [-3, 3, -3, 3])
         np.testing.assert_array_equal(expand_dims(-3), [-3, 3, -3, 3])
@@ -56,7 +82,9 @@ def test_expand_dims(self):
 
         np.testing.assert_array_equal(expand_dims(-3, nd=3), [-3, 3, -3, 3, -3, 3])
         np.testing.assert_array_equal(expand_dims([1, 2], nd=3), [1, 2] * 3)
-        np.testing.assert_array_equal(expand_dims([1, 2, 3, 4, 5, 6], nd=3), [1, 2, 3, 4, 5, 6])
+        np.testing.assert_array_equal(
+            expand_dims([1, 2, 3, 4, 5, 6], nd=3), [1, 2, 3, 4, 5, 6]
+        )
 
         with self.assertRaises(ValueError):
             expand_dims([1, 2, 3])