Use the cube dimension order to determine the plot axes.

docs/iris/src/whatsnew/contributions_2.0/incompatiblechange_2017-Oct-30_axis_order_plotting.txt

@@ -0,0 +1 @@
+* When coordinates have no well defined plot axis, iris.plot and iris.quickplot routines now use the order of the cube's dimensions to determine the coordinates to plot as the x and y axis of a plot. 

lib/iris/plot.py

@@ -186,11 +186,15 @@ def guess_axis(coord):
                     coords[axes.index(axis)] = coord
 
     # Re-order the coordinates to achieve the preferred
-    # horizontal/vertical associations.
+    # horizontal/vertical associations. If we can't associate
+    # an axis to order the coordinates, fall back to using the cube dimension
+    # followed by the name of the coordinate.
     def sort_key(coord):
         order = {'X': 2, 'T': 1, 'Y': -1, 'Z': -2}
         axis = guess_axis(coord)
-        return (order.get(axis, 0), coord and coord.name())
+        return (order.get(axis, 0),
+                coords.index(coord),
+                coord and coord.name())
     sorted_coords = sorted(coords, key=sort_key)
 
     transpose = (sorted_coords != coords)

lib/iris/tests/test_mapping.py

@@ -102,7 +102,7 @@ def setUp(self):
         self.cube = cube
 
     def test_simple(self):
-        iplt.contourf(self.cube)
+        iplt.contourf(self.cube, coords=['y', 'x'])
         self.check_graphic()
 
 

lib/iris/tests/unit/plot/test__get_plot_defn.py

@@ -0,0 +1,50 @@
+# (C) British Crown Copyright 2017, Met Office
+#
+# This file is part of Iris.
+#
+# Iris is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Lesser General Public License as published by the
+# Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Iris is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with Iris.  If not, see <http://www.gnu.org/licenses/>.
+"""Unit tests for the `iris.plot._get_plot_defn` function."""
+
+from __future__ import (absolute_import, division, print_function)
+from six.moves import (filter, input, map, range, zip)  # noqa
+
+# Import iris.tests first so that some things can be initialised before
+# importing anything else.
+import iris.tests as tests
+
+import iris.coords
+from iris.tests.stock import simple_2d
+
+if tests.MPL_AVAILABLE:
+    import iris.plot as iplt
+
+
+@tests.skip_plot
+class Test_get_plot_defn(tests.IrisTest):
+    def test_axis_order_xy(self):
+        cube_xy = simple_2d()
+        defn = iplt._get_plot_defn(cube_xy, iris.coords.POINT_MODE)
+        self.assertEqual([coord.name() for coord in defn.coords],
+                         ['bar', 'foo'])
+
+    def test_axis_order_yx(self):
+        cube_yx = simple_2d()
+        cube_yx.transpose()
+        defn = iplt._get_plot_defn(cube_yx, iris.coords.POINT_MODE)
+        self.assertEqual([coord.name() for coord in defn.coords],
+                         ['foo', 'bar'])
+
+
+if __name__ == "__main__":
+    tests.main()