Merge pull request matplotlib#24904 from anntzer/aaf

Deprecate AxisArtistHelpers with inconsistent loc/nth_coord.

Author: oscargus

doc/api/next_api_changes/deprecations/24904-AL.rst

@@ -0,0 +1,4 @@
+Passing inconsistent ``loc`` and ``nth_coord`` to axisartist helpers
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Trying to construct for example a "top y-axis" or a "left x-axis" is now
+deprecated.

lib/mpl_toolkits/axisartist/axislines.py

@@ -126,12 +126,17 @@ class Fixed(_Base):
 
         def __init__(self, loc, nth_coord=None):
             """``nth_coord = 0``: x-axis; ``nth_coord = 1``: y-axis."""
-            _api.check_in_list(["left", "right", "bottom", "top"], loc=loc)
-            self._loc = loc
-            self._pos = {"bottom": 0, "top": 1, "left": 0, "right": 1}[loc]
             self.nth_coord = (
                 nth_coord if nth_coord is not None else
-                {"bottom": 0, "top": 0, "left": 1, "right": 1}[loc])
+                _api.check_getitem(
+                    {"bottom": 0, "top": 0, "left": 1, "right": 1}, loc=loc))
+            if (nth_coord == 0 and loc not in ["left", "right"]
+                    or nth_coord == 1 and loc not in ["bottom", "top"]):
+                _api.warn_deprecated(
+                    "3.7", message=f"{loc=!r} is incompatible with "
+                    "{nth_coord=}; support is deprecated since %(since)s")
+            self._loc = loc
+            self._pos = {"bottom": 0, "top": 1, "left": 0, "right": 1}[loc]
             super().__init__()
             # axis line in transAxes
             self._path = Path(self._to_xy((0, 1), const=self._pos))