Merge pull request #97 from esc24/exner_fix

Axis unit labels

docs/iris/example_code/graphics/deriving_phenomena.py

@@ -2,11 +2,11 @@
 Deriving Exner Pressure and Air Temperature
 ===========================================
 
-This example shows some processing of cubes in order to derive further related cubes; in this case
-the derived cubes are Exner pressure and air temperature which are calculated by combining air pressure,
-air potential temperature and specific humidity. Finally, the two new cubes are presented side-by-side in
-a plot.
-
+This example shows some processing of cubes in order to derive further related
+cubes; in this case the derived cubes are Exner pressure and air temperature
+which are calculated by combining air pressure, air potential temperature and
+specific humidity. Finally, the two new cubes are presented side-by-side in a
+plot.
 
 """
 import itertools
@@ -15,11 +15,16 @@
 
 import iris
 import iris.coords as coords
+import iris.iterate
 import iris.quickplot as qplt
 
 
 def limit_colorbar_ticks(contour_object):
-    """Takes a contour object which has an associated colorbar and limits the number of ticks on the colorbar to 4."""
+    """
+    Takes a contour object which has an associated colorbar and limits the
+    number of ticks on the colorbar to 4.
+
+    """
     colorbar = contour_object.colorbar[0]
     colorbar.locator = matplotlib.ticker.MaxNLocator(4)
     colorbar.update_ticks()
@@ -28,52 +33,53 @@ def limit_colorbar_ticks(contour_object):
 def main():
     fname = iris.sample_data_path('colpex.pp')
 
-    # the list of phenomena of interest
+    # The list of phenomena of interest
     phenomena = ['air_potential_temperature', 'air_pressure']
 
-    # define the constraint on standard name and model level
-    constraints = [iris.Constraint(phenom, model_level_number=1) for phenom in phenomena]
+    # Define the constraint on standard name and model level
+    constraints = [iris.Constraint(phenom, model_level_number=1) for
+                   phenom in phenomena]
 
     air_potential_temperature, air_pressure = iris.load_cubes(fname,
                                                               constraints) 
 
-    # define a coordinate which represents 1000 hPa 
+    # Define a coordinate which represents 1000 hPa
     p0 = coords.AuxCoord(100000, long_name='P0', units='Pa')
 
-    # calculate Exner pressure
+    # Calculate Exner pressure
     exner_pressure = (air_pressure / p0) ** (287.05 / 1005.0)
-    # set the standard name (the unit is scalar)
+    # Set the name (the unit is scalar)
     exner_pressure.rename('exner_pressure')
 
-
-    # calculate air_temp
+    # Calculate air_temp
     air_temperature = exner_pressure * air_potential_temperature
-    # set phenomenon definition and unit
-    air_temperature.standard_name = 'air_temperature'
-    air_temperature.units = 'K'
+    # Set the name (the unit is K)
+    air_temperature.rename('air_temperature')
 
-
-    # Now create an iterator which will give us lat lon slices of exner pressure and air temperature in
-    # the form [exner_slice, air_temp_slice]
-    lat_lon_slice_pairs = itertools.izip(
-                                         exner_pressure.slices(['grid_latitude', 'grid_longitude']), 
-                                         air_temperature.slices(['grid_latitude', 'grid_longitude'])
-                                         )
-    plt.figure(figsize=(8, 4))
+    # Now create an iterator which will give us lat lon slices of
+    # exner pressure and air temperature in the form
+    # (exner_slice, air_temp_slice).
+    lat_lon_slice_pairs = iris.iterate.izip(exner_pressure,
+                                            air_temperature,
+                                            coords=['grid_latitude',
+                                                    'grid_longitude'])
 
+    plt.figure(figsize=(8, 4))
     for exner_slice, air_temp_slice in lat_lon_slice_pairs:
         plt.subplot(121)
         cont = qplt.contourf(exner_slice)
 
-        # The default colorbar has a few too many ticks on it, causing text to overlap. Therefore, limit the number of ticks
+        # The default colorbar has a few too many ticks on it, causing text to
+        # overlap. Therefore, limit the number of ticks.
         limit_colorbar_ticks(cont)
 
         plt.subplot(122)
         cont = qplt.contourf(air_temp_slice)
         limit_colorbar_ticks(cont)
         plt.show()
 
-        # For the purposes of this example, break after the first loop - we only want to demonstrate the first plot
+        # For the purposes of this example, break after the first loop - we
+        # only want to demonstrate the first plot.
         break
 
 

lib/iris/quickplot.py

@@ -36,8 +36,19 @@ def _title(cube_or_coord, with_units):
         title = ''
     else:
         title = cube_or_coord.name().replace('_', ' ').capitalize()
-        if with_units:
-            title += ' / {}'.format(cube_or_coord.units)
+        units = cube_or_coord.units
+        if with_units and not (units.unknown or
+                               units.no_unit or
+                               units == iris.unit.Unit('1')):
+
+            # For non-time units use the shortest unit representation e.g.
+            # prefer 'K' over 'kelvin', but not '0.0174532925199433 rad'
+            # over 'degrees'
+            if (not units.is_time() and not units.time_reference and
+                len(units.symbol) < len(str(units))):
+                units = units.symbol
+            title += ' / {}'.format(units)
+
     return title
 
 
@@ -48,8 +59,16 @@ def _label(cube, mode, result=None, ndims=2, coords=None):
 
     if result is not None:
         draw_edges = mode == iris.coords.POINT_MODE
-        bar = plt.colorbar(result, orientation='horizontal', drawedges=draw_edges)
-        bar.set_label(cube.units)
+        bar = plt.colorbar(result, orientation='horizontal',
+                           drawedges=draw_edges)
+        has_known_units = not (cube.units.unknown or cube.units.no_unit)
+        if has_known_units and cube.units != iris.unit.Unit('1'):
+            # Use shortest unit representation for anything other than time
+            if (not cube.units.is_time() and not cube.units.time_reference and
+                len(cube.units.symbol) < len(str(cube.units))):
+                bar.set_label(cube.units.symbol)
+            else:
+                bar.set_label(cube.units)
         # Remove the tick which is put on the colorbar by default.
         bar.ax.tick_params(length=0)