BUG: Fix face orientations of bar3d (matplotlib#12259)

* STY: Add trailing commas

This makes the next commit easier to follow

* BUG: Fix face orientations of bar3d

Fixes matplotlib#12138, which is caused by these incorrect orientations.

This also corrects _generate_normals to use a counterclockwise convention

Author: eric-wieser

lib/mpl_toolkits/mplot3d/axes3d.py

@@ -1736,11 +1736,14 @@ def _generate_normals(self, polygons):
         Generate normals for polygons by using the first three points.
         This normal of course might not make sense for polygons with
         more than three points not lying in a plane.
+
+        Normals point towards the viewer for a face with its vertices in
+        counterclockwise order, following the right hand rule.
         '''
 
         normals = []
         for verts in polygons:
-            v1 = np.array(verts[0]) - np.array(verts[1])
+            v1 = np.array(verts[1]) - np.array(verts[0])
             v2 = np.array(verts[2]) - np.array(verts[0])
             normals.append(np.cross(v1, v2))
         return normals
@@ -2452,48 +2455,50 @@ def bar3d(self, x, y, z, dx, dy, dz, color=None,
         maxz = np.max(z + dz)
 
         # shape (6, 4, 3)
+        # All faces are oriented facing outwards - when viewed from the
+        # outside, their vertices are in a counterclockwise ordering.
         cuboid = np.array([
             # -z
             (
                 (0, 0, 0),
-                (1, 0, 0),
+                (0, 1, 0),
                 (1, 1, 0),
-                (0, 1, 0)
+                (1, 0, 0),
             ),
             # +z
             (
                 (0, 0, 1),
                 (1, 0, 1),
                 (1, 1, 1),
-                (0, 1, 1)
+                (0, 1, 1),
             ),
             # -y
             (
                 (0, 0, 0),
                 (1, 0, 0),
                 (1, 0, 1),
-                (0, 0, 1)
+                (0, 0, 1),
             ),
             # +y
             (
                 (0, 1, 0),
-                (1, 1, 0),
+                (0, 1, 1),
                 (1, 1, 1),
-                (0, 1, 1)
+                (1, 1, 0),
             ),
             # -x
             (
                 (0, 0, 0),
-                (0, 1, 0),
+                (0, 0, 1),
                 (0, 1, 1),
-                (0, 0, 1)
+                (0, 1, 0),
             ),
             # +x
             (
                 (1, 0, 0),
                 (1, 1, 0),
                 (1, 1, 1),
-                (1, 0, 1)
+                (1, 0, 1),
             ),
         ])