Wind barb plotting bug fix

Handle arrays of Pint objects when plotting wind barbs

src/metpy/plots/station_plot.py

@@ -10,6 +10,7 @@
 from .wx_symbols import (current_weather, high_clouds, low_clouds, mid_clouds,
                          pressure_tendency, sky_cover, wx_symbol_font)
 from ..package_tools import Exporter
+from ..units import units
 
 exporter = Exporter(globals())
 
@@ -328,6 +329,12 @@ def plot_arrow(self, u, v, **kwargs):
     @staticmethod
     def _vector_plotting_units(u, v, plotting_units):
         """Handle conversion to plotting units for barbs and arrows."""
+        # If u and v are arrays of Pint objects, put units outside array
+        if hasattr(u[0], 'units'):
+            u = units.Quantity(np.fromiter((obj.magnitude for obj in u), float), u[0].units)
+        if hasattr(v[0], 'units'):
+            v = units.Quantity(np.fromiter((obj.magnitude for obj in v), float), v[0].units)
+
         if plotting_units:
             if hasattr(u, 'units') and hasattr(v, 'units'):
                 u = u.to(plotting_units)

tests/plots/test_station_plot.py

@@ -371,6 +371,24 @@ def test_barb_unit_conversion(barbs_units):
     return fig
 
 
+@pytest.mark.mpl_image_compare(tolerance=0.0048, remove_text=True)
+def test_barb_handling_arrays_of_pint_objects(barbs_units):
+    """Test handling u and v that are arrays of Pint quantities."""
+    x_pos = np.array([1])
+    y_pos = np.array([1])
+    u_wind = np.array([2 * units.knot], dtype='object')
+    v_wind = np.array([-4 * units.knot], dtype='object')
+
+    fig = plt.figure()
+    ax = fig.add_subplot(1, 1, 1)
+    stnplot = StationPlot(ax, x_pos, y_pos)
+    stnplot.plot_barb(u_wind, v_wind, plot_units='knots')
+    ax.set_xlim(-5, 5)
+    ax.set_ylim(-5, 5)
+
+    return fig
+
+
 @pytest.mark.mpl_image_compare(tolerance=0.0048, remove_text=True)
 def test_arrow_unit_conversion(barbs_units):
     """Test that arrow units can be converted at plot time (#737)."""