Fix circular quiver plots

lib/iris/plot.py

@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2010 - 2018, Met Office
+# (C) British Crown Copyright 2010 - 2019, Met Office
 #
 # This file is part of Iris.
 #
@@ -873,6 +873,10 @@ def _map_common(draw_method_name, arg_func, mode, cube, plot_defn,
         y = np.append(y, y[:, 0:1], axis=1)
         x = np.append(x, x[:, 0:1] + 360 * direction, axis=1)
         data = ma.concatenate([data, data[:, 0:1]], axis=1)
+        if '_v_data' in kwargs:
+            v_data = kwargs['_v_data']
+            v_data = ma.concatenate([v_data, v_data[:, 0:1]], axis=1)
+            kwargs['_v_data'] = v_data
 
     # Replace non-cartopy subplot/axes with a cartopy alternative and set the
     # transform keyword.

lib/iris/tests/integration/plot/test_vector_plots.py

@@ -167,6 +167,26 @@ def test_fail_unsupported_coord_system(self):
             self.plot('2d_rotated', u_cube, v_cube,
                       coords=('longitude', 'latitude'))
 
+    def test_circular_longitude(self):
+        # Test circular longitude does not cause a crash.
+        res = 5
+        lat = DimCoord(np.arange(-90, 91, res), 'latitude',
+                                   units='degrees_north')
+        lon = DimCoord(np.arange(0, 360, res), 'longitude',
+                                   units='degrees_east')
+        nlat = len(lat.points)
+        nlon = len(lon.points)
+        u_arr = np.ones((nlat, nlon))
+        v_arr = np.ones((nlat, nlon))
+        u_cube = Cube(u_arr, dim_coords_and_dims=[(lat, 0), (lon, 1)],
+                           standard_name='eastward_wind')
+        v_cube = Cube(v_arr, dim_coords_and_dims=[(lat, 0), (lon, 1)],
+                           standard_name='northward_wind')
+        u_cube.coord('longitude').circular = True
+
+        self.plot('circular', u_cube, v_cube,
+                  coords=('longitude', 'latitude'))
+
 
 class TestQuiver(MixinVectorPlotCases, tests.GraphicsTest):
     def setUp(self):