try adding a simple plot with ipython directives

Author: wholmgren

docs/environment.yml

@@ -10,4 +10,5 @@ dependencies:
     - ipython
     - sphinx
     - numpydoc
+    - matplotlib
     - seaborn

pvlib/irradiance.py

@@ -87,6 +87,19 @@ def extraradiation(datetime_or_doy, solar_constant=1366.1, method='spencer'):
     See Also
     --------
     pvlib.clearsky.disc
+    
+    Examples
+    --------
+    .. ipython::
+        In [1]: times = pd.date_range('2014-01-01', '2015-01-01', freq='1D')
+        In [2]: spencer = pd.Series(pvlib.irradiance.extraradiation(times, method='spencer'), times)
+                asce = pd.Series(pvlib.irradiance.extraradiation(times, method='asce'), times)
+                ephem = pvlib.irradiance.extraradiation(times, method='pyephem') # approx 100x slower than the above.
+        In [3]: spencer.plot(label='spencer')
+                asce.plot(label='asce')
+                ephem.plot(label='pyephem')
+                plt.legend()
+                plt.ylabel('Extraterrestrial radiation (W/m^2)')
     """
 
     pvl_logger.debug('irradiance.extraradiation()')