Preserve position when replacing Axes with GeoAxes

docs/src/whatsnew/dev.rst

@@ -41,14 +41,18 @@ This document explains the changes made to Iris for this release
 #. `@rcomer`_ reverted part of the change from :pull:`3906` so that
    :func:`iris.plot.plot` no longer defaults to placing a "Y" coordinate (e.g.
    latitude) on the y-axis of the plot. (:issue:`4493`, :pull:`4601`)
-   
-#. `@rcomer`_ enabled passing of scalar objects to :func:`~iris.plot.plot` and 
+
+#. `@rcomer`_ enabled passing of scalar objects to :func:`~iris.plot.plot` and
    :func:`~iris.plot.scatter`. (:pull:`4616`)
 
 #. `@rcomer`_ fixed :meth:`~iris.cube.Cube.aggregated_by` with `mdtol` for 1D
    cubes where an aggregated section is entirely masked, reported at
    :issue:`3190`.  (:pull:`4246`)
 
+#. `@rcomer`_ ensured that a :class:`matplotlib.axes.Axes`'s position is preserved
+   when Iris replaces it with a :class:`cartopy.mpl.geoaxes.GeoAxes`, fixing
+   :issue:`1157`.  (:pull:`4273`)
+
 
 💣 Incompatible Changes
 =======================

lib/iris/plot.py

@@ -843,7 +843,9 @@ def _replace_axes_with_cartopy_axes(cartopy_proj):
                 ylabel=ax.get_ylabel(),
             )
         else:
+            position = ax.get_position(original=True)
             _ = fig.add_axes(
+                position,
                 projection=cartopy_proj,
                 title=ax.get_title(),
                 xlabel=ax.get_xlabel(),

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

@@ -0,0 +1,45 @@
+# Copyright Iris contributors
+#
+# This file is part of Iris and is released under the LGPL license.
+# See COPYING and COPYING.LESSER in the root of the repository for full
+# licensing details.
+"""Unit tests for the `iris.plot.__replace_axes_with_cartopy_axes` function."""
+
+# Import iris.tests first so that some things can be initialised before
+# importing anything else.
+import iris.tests as tests  # isort:skip
+
+import cartopy.crs as ccrs
+import matplotlib.pyplot as plt
+
+from iris.plot import _replace_axes_with_cartopy_axes
+
+
+@tests.skip_plot
+class Test_replace_axes_with_cartopy_axes(tests.IrisTest):
+    def setUp(self):
+        self.fig = plt.figure()
+
+    def test_preserve_position(self):
+        position = [0.17, 0.65, 0.2, 0.2]
+        projection = ccrs.PlateCarree()
+
+        plt.axes(position)
+        _replace_axes_with_cartopy_axes(projection)
+        result = plt.gca()
+
+        # result should be the same as an axes created directly with the projection.
+        expected = plt.axes(position, projection=projection)
+
+        # get_position returns mpl.transforms.Bbox object, for which equality does
+        # not appear to be implemented.  Compare the bounds (tuple) instead.
+        self.assertEqual(
+            expected.get_position().bounds, result.get_position().bounds
+        )
+
+    def tearDown(self):
+        plt.close(self.fig)
+
+
+if __name__ == "__main__":
+    tests.main()