fix(graphics): ignore sign of scalar in expand_dims, document reversed limits (#262)

A negative scalar passed to `expand_dims` (and so
`plotvol2`/`plotvol3`/`Animate`) gave reversed limits, e.g. `-3` -> `[3,
-3, 3, -3]`, silently inverting the axes. A scalar is symmetric about
zero, so the sign is now ignored via `abs()`.

For the 2/4/6 element forms no min < max check is added. Matplotlib
inverts an axis when given reversed limits, so this is a useful way to
flip an axis, and `expand_dims` is a pure function with no axes to call
`invert_xaxis()` on. The docstring now says so.

Adds `test_expand_dims`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

Author: petercorke

spatialmath/base/graphics.py

@@ -1759,11 +1759,18 @@ def expand_dims(dim: ArrayLike = None, nd: int = 2) -> NDArray:
             * A -> [-A, A, -A, A, -A, A]
             * [A,B] -> [A, B, A, B, A, B]
             * [A,B,C,D,E,F] -> [A, B, C, D, E, F]
+
+        A scalar is symmetric about zero, so its sign is ignored: ``-A`` is the
+        same as ``A``.  For the other forms the values are returned as given and
+        no ordering check is made.  If a minimum is greater than the
+        corresponding maximum, matplotlib will invert that axis when the limits
+        are applied, which is a convenient way to flip an axis direction.
         """
         dim = smb.getvector(dim)
 
         if nd == 2:
             if len(dim) == 1:
+                dim = abs(dim)
                 return np.r_[-dim, dim, -dim, dim]
             elif len(dim) == 2:
                 return np.r_[dim[0], dim[1], dim[0], dim[1]]
@@ -1773,6 +1780,7 @@ def expand_dims(dim: ArrayLike = None, nd: int = 2) -> NDArray:
                 raise ValueError("bad dimension specified")
         elif nd == 3:
             if len(dim) == 1:
+                dim = abs(dim)
                 return np.r_[-dim, dim, -dim, dim, -dim, dim]
             elif len(dim) == 2:
                 return np.r_[dim[0], dim[1], dim[0], dim[1], dim[0], dim[1]]

tests/base/test_graphics.py

@@ -13,6 +13,23 @@ class TestGraphics(unittest.TestCase):
     def teardown_method(self, method):
         plt.close("all")
 
+    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])
+        np.testing.assert_array_equal(expand_dims([1, 2]), [1, 2, 1, 2])
+        np.testing.assert_array_equal(expand_dims([1, 2, 3, 4]), [1, 2, 3, 4])
+        # reversed limits are passed through, matplotlib inverts the axis
+        np.testing.assert_array_equal(expand_dims([2, 1]), [2, 1, 2, 1])
+
+        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])
+
+        with self.assertRaises(ValueError):
+            expand_dims([1, 2, 3])
+        with self.assertRaises(ValueError):
+            expand_dims([1, 2, 3, 4], nd=3)
+
     @pytest.mark.skipif(
         sys.platform.startswith("darwin") and sys.version_info < (3, 11),
         reason="tkinter bug with mac",