FIX: double z-axis draw in mplot3D

When we moved `Axes.draw` to use `Axes.get_children` to get the initial
list of artists to draw the zaxis was now in this list (where as it was
not previously).  The 3D axes use `_axison = False` as `Axes3D` manages
the drawing of the axis objects (which must happen before any of the
artists). In `Axes.draw` there is a special case to remove the x and y
axis from the draw list if `not _axison`.

This change is to add a `_get_axis_list` method to the `Axes` base class
and override this in the `Axes3D`.  This list is looped over to remove
all of the `axis` objects that when the axises should not be shown.

Closes matplotlib#4971

lib/matplotlib/axes/_base.py

@@ -2049,6 +2049,9 @@ def autoscale_view(self, tight=None, scalex=True, scaley=True):
                 y0, y1 = ylocator.view_limits(y0, y1)
             self.set_ybound(y0, y1)
 
+    def _get_axis_list(self):
+        return (self.xaxis, self.yaxis)
+
     # Drawing
 
     @allow_rasterization
@@ -2090,8 +2093,8 @@ def draw(self, renderer=None, inframe=False):
                 self.xaxis.set_zorder(2.5)
                 self.yaxis.set_zorder(2.5)
         else:
-            artists.remove(self.xaxis)
-            artists.remove(self.yaxis)
+            for _axis in self._get_axis_list():
+                artists.remove(_axis)
 
         if inframe:
             artists.remove(self.title)

lib/mpl_toolkits/mplot3d/axes3d.py

@@ -192,26 +192,29 @@ def set_top_view(self):
     def _init_axis(self):
         '''Init 3D axes; overrides creation of regular X/Y axes'''
         self.w_xaxis = axis3d.XAxis('x', self.xy_viewLim.intervalx,
-                            self.xy_dataLim.intervalx, self)
+                                    self.xy_dataLim.intervalx, self)
         self.xaxis = self.w_xaxis
         self.w_yaxis = axis3d.YAxis('y', self.xy_viewLim.intervaly,
-                            self.xy_dataLim.intervaly, self)
+                                    self.xy_dataLim.intervaly, self)
         self.yaxis = self.w_yaxis
         self.w_zaxis = axis3d.ZAxis('z', self.zz_viewLim.intervalx,
-                            self.zz_dataLim.intervalx, self)
+                                    self.zz_dataLim.intervalx, self)
         self.zaxis = self.w_zaxis
 
         for ax in self.xaxis, self.yaxis, self.zaxis:
             ax.init3d()
 
     def get_children(self):
-        return [self.zaxis,] + Axes.get_children(self)
+        return [self.zaxis, ] + Axes.get_children(self)
+
+    def _get_axis_list(self):
+        return super(Axes3D, self)._get_axis_list() + (self.zaxis, )
 
     def unit_cube(self, vals=None):
         minx, maxx, miny, maxy, minz, maxz = vals or self.get_w_lims()
         xs, ys, zs = ([minx, maxx, maxx, minx, minx, maxx, maxx, minx],
-                    [miny, miny, maxy, maxy, miny, miny, maxy, maxy],
-                    [minz, minz, minz, minz, maxz, maxz, maxz, maxz])
+                      [miny, miny, maxy, maxy, miny, miny, maxy, maxy],
+                      [minz, minz, minz, minz, maxz, maxz, maxz, maxz])
         return list(zip(xs, ys, zs))
 
     def tunit_cube(self, vals=None, M=None):
@@ -1718,7 +1721,7 @@ def plot_wireframe(self, X, Y, Z, *args, **kwargs):
 
         The `rstride` and `cstride` kwargs set the stride used to
         sample the input data to generate the graph. If either is 0
-        the input data in not sampled along this direction producing a 
+        the input data in not sampled along this direction producing a
         3D line plot rather than a wireframe plot.
 
         ==========  ================================================
@@ -2438,7 +2441,7 @@ def bar3d(self, x, y, z, dx, dy, dz, color='b',
         self.add_collection(col)
 
         self.auto_scale_xyz((minx, maxx), (miny, maxy), (minz, maxz), had_data)
-        
+
         return col
 
     def set_title(self, label, fontdict=None, loc='center', **kwargs):