Merge pull request apache#92 from john-bodley/john-bodley-cherry-pick…

…-4964

Fix edge case around NaN values (apache#4964)

superset/viz.py

@@ -217,7 +217,7 @@ def df_metrics_to_num(df, metrics):
         """Converting metrics to numeric when pandas.read_sql cannot"""
         for col, dtype in df.dtypes.items():
             if dtype.type == np.object_ and col in metrics:
-                df[col] = pd.to_numeric(df[col])
+                df[col] = pd.to_numeric(df[col], errors='coerce')
 
     def query_obj(self):
         """Building a query object"""