add comment per @vshnayder comment on pull #588

common/djangoapps/xmodule_modifiers.py

@@ -75,7 +75,7 @@ def grade_histogram(module_id):
 
     grades = list(cursor.fetchall())
     grades.sort(key=lambda x: x[0])          # Add ORDER BY to sql query?
-    if len(grades) >= 1 and grades[0][0] is None:
+    if len(grades) >= 1 and grades[0][0] is None:	# list of [grade, count] pairs
         return []
     return grades