quickplot: locate colourbar with axes if supplied

docs/src/whatsnew/latest.rst

@@ -65,6 +65,9 @@ This document explains the changes made to Iris for this release
 #. `@rcomer`_ modified :func:`~iris.plot.contourf` to skip the special handling for
    antialiasing when data values are too low for it to have an effect.  This caused
    unexpected artifacts in some edge cases, as shown at :issue:`4086`. (:pull:`4150`)
+
+#. `@rcomer`_ modified :mod:`iris.quickplot` so that, if an axes is supplied, any
+   colourbar will appear below it, fixing :issue:`4126`.  (:pull:`4136`)
 
 
 💣 Incompatible Changes

lib/iris/quickplot.py

@@ -72,7 +72,7 @@ def _label(cube, mode, result=None, ndims=2, coords=None, axes=None):
     if result is not None:
         draw_edges = mode == iris.coords.POINT_MODE
         bar = plt.colorbar(
-            result, orientation="horizontal", drawedges=draw_edges
+            result, orientation="horizontal", drawedges=draw_edges, ax=axes
         )
         has_known_units = not (
             cube.units.is_unknown() or cube.units.is_no_unit()

lib/iris/tests/test_quickplot.py

@@ -247,5 +247,24 @@ def test_not_reference_time_units(self):
         self.check_graphic()
 
 
+@tests.skip_data
+@tests.skip_plot
+class TestColorbar(tests.IrisTest):
+    def test_colorbar_location(self):
+        theta = _load_theta()
+        theta_slice = next(
+            theta.slices(["model_level_number", "grid_longitude"])
+        )
+
+        fig1 = plt.figure()
+        plt.figure()
+        ax = fig1.add_subplot(1, 1, 1)
+
+        result = qplt.contourf(theta_slice, axes=ax)
+
+        # Colourbar should appear on same figure as ax, i.e. fig1.
+        self.assertIs(result.colorbar.ax.get_figure(), fig1)
+
+
 if __name__ == "__main__":
     tests.main()