Move restriction of polar theta scales to ThetaAxis._set_scale.

This ensures that there's a single place handling the restriction of
theta scales, which makes it simpler to follow (otherwise, in
ThetaAxis._set_scale, it's not clear what values can be taken by
`value`).

Author: anntzer

lib/matplotlib/projections/polar.py

@@ -389,6 +389,9 @@ def cla(self):
         self.clear()
 
     def _set_scale(self, value, **kwargs):
+        if value != 'linear':
+            raise NotImplementedError(
+                "The xscale cannot be set on a polar plot")
         super()._set_scale(value, **kwargs)
         self._wrap_locator_formatter()
 
@@ -1391,11 +1394,6 @@ def set_rgrids(self, radii, labels=None, angle=None, fmt=None, **kwargs):
             t.update(kwargs)
         return self.yaxis.get_gridlines(), self.yaxis.get_ticklabels()
 
-    def set_xscale(self, scale, *args, **kwargs):
-        if scale != 'linear':
-            raise NotImplementedError(
-                "You can not set the xscale on a polar plot.")
-
     def format_coord(self, theta, r):
         # docstring inherited
         if theta < 0: