More readability improvements on axis3d.

At least the comment helps me, and whole thing is shorter :)

Author: anntzer

lib/mpl_toolkits/mplot3d/axis3d.py

@@ -283,8 +283,7 @@ def draw(self, renderer):
             outeredgep = edgep2
             outerindex = 1
 
-        pos = outeredgep.copy()
-        pos = move_from_center(pos, centers, labeldeltas, axmask)
+        pos = move_from_center(outeredgep, centers, labeldeltas, axmask)
         olx, oly, olz = proj3d.proj_transform(*pos, renderer.M)
         self.offsetText.set_text(self.major.formatter.get_offset())
         self.offsetText.set_position((olx, oly))
@@ -339,19 +338,14 @@ def draw(self, renderer):
         self.offsetText.set_ha(align)
         self.offsetText.draw(renderer)
 
-        # Draw grid lines
         if self.axes._draw_grid and len(ticks):
-            # Grid points at end of one plane
-            xyz1 = xyz0.copy()
-            newindex = (index + 1) % 3
-            xyz1[:, newindex] = maxmin[newindex]
-
-            # Grid points at end of the other plane
-            xyz2 = xyz0.copy()
-            newindex = (index + 2) % 3
-            xyz2[:, newindex] = maxmin[newindex]
-
-            lines = np.stack([xyz1, xyz0, xyz2], axis=1)
+            # Grid lines go from the end of one plane through the plane
+            # intersection (at xyz0) to the end of the other plane.  The first
+            # point (0) differs along dimension index-2 and the last (2) along
+            # dimension index-1.
+            lines = np.stack([xyz0, xyz0, xyz0], axis=1)
+            lines[:, 0, index - 2] = maxmin[index - 2]
+            lines[:, 2, index - 1] = maxmin[index - 1]
             self.gridlines.set_segments(lines)
             self.gridlines.set_color(info['grid']['color'])
             self.gridlines.set_linewidth(info['grid']['linewidth'])