Add declarative attribute plot_units

src/metpy/plots/declarative.py

@@ -783,6 +783,13 @@ class Plots2D(HasTraits):
     from the Python standard library.
     """
 
+    plot_units = Unicode(allow_none=True, default_value=None)
+    plot_units.__doc__ = """The desired units to plot the field in.
+
+    Setting this attribute will convert the units of the field variable to the given units for
+    plotting using the MetPy Units module.
+    """
+
     @property
     def _cmap_obj(self):
         """Return the colormap object.
@@ -918,7 +925,11 @@ def griddata(self):
 
             if self.time is not None:
                 subset[data.metpy.time.name] = self.time
-            self._griddata = data.metpy.sel(**subset).squeeze()
+            data_subset = data.metpy.sel(**subset).squeeze()
+
+            if self.plot_units is not None:
+                data_subset.metpy.convert_units(self.plot_units)
+            self._griddata = data_subset
 
         return self._griddata
 
@@ -1191,8 +1202,14 @@ def griddata(self):
 
             if self.time is not None:
                 subset[u.metpy.time.name] = self.time
-            self._griddata_u = u.metpy.sel(**subset).squeeze()
-            self._griddata_v = v.metpy.sel(**subset).squeeze()
+            data_subset_u = u.metpy.sel(**subset).squeeze()
+            data_subset_v = v.metpy.sel(**subset).squeeze()
+
+            if self.plot_units is not None:
+                data_subset_u.metpy.convert_units(self.plot_units)
+                data_subset_v.metpy.convert_units(self.plot_units)
+            self._griddata_u = data_subset_u
+            self._griddata_v = data_subset_v
 
         return (self._griddata_u, self._griddata_v)
 

tests/plots/test_declarative.py

@@ -106,6 +106,36 @@ def test_declarative_contour_options():
     return pc.figure
 
 
+@pytest.mark.mpl_image_compare(remove_text=True, tolerance=0.035)
+def test_declarative_contour_convert_units():
+    """Test making a contour plot."""
+    data = xr.open_dataset(get_test_data('narr_example.nc', as_file_obj=False))
+
+    contour = ContourPlot()
+    contour.data = data
+    contour.field = 'Temperature'
+    contour.level = 700 * units.hPa
+    contour.contours = 30
+    contour.linewidth = 1
+    contour.linecolor = 'red'
+    contour.linestyle = 'dashed'
+    contour.clabels = True
+    contour.plot_units = 'degC'
+
+    panel = MapPanel()
+    panel.area = 'us'
+    panel.proj = 'lcc'
+    panel.layers = ['coastline', 'borders', 'usstates']
+    panel.plots = [contour]
+
+    pc = PanelContainer()
+    pc.size = (8, 8)
+    pc.panels = [panel]
+    pc.draw()
+
+    return pc.figure
+
+
 @pytest.mark.mpl_image_compare(remove_text=True, tolerance=0.016)
 def test_declarative_events():
     """Test that resetting traitlets properly propagates."""
@@ -417,6 +447,33 @@ def test_declarative_barb_gfs():
     return pc.figure
 
 
+@pytest.mark.mpl_image_compare(remove_text=True, tolerance=0.346)
+def test_declarative_barb_gfs_knots():
+    """Test making a contour plot."""
+    data = xr.open_dataset(get_test_data('GFS_test.nc', as_file_obj=False))
+
+    barb = BarbPlot()
+    barb.data = data
+    barb.level = 300 * units.hPa
+    barb.field = ['u-component_of_wind_isobaric', 'v-component_of_wind_isobaric']
+    barb.skip = (3, 3)
+    barb.earth_relative = False
+    barb.plot_units = 'knot'
+
+    panel = MapPanel()
+    panel.area = 'us'
+    panel.projection = 'data'
+    panel.layers = ['coastline', 'borders', 'usstates']
+    panel.plots = [barb]
+
+    pc = PanelContainer()
+    pc.size = (8, 8)
+    pc.panels = [panel]
+    pc.draw()
+
+    return pc.figure
+
+
 @pytest.mark.mpl_image_compare(remove_text=True, tolerance=0.022)
 def test_declarative_sfc_obs():
     """Test making a surface observation plot."""