On the topic, I was mildly interested to see that the action chain works on a custom ratings column (though sum and median aren't very useful in this case):
Code:
Column name: Ratings (#ratings)
Column type: rating
Count of selected books: 635
Count of books with a non-empty value: 635
Sum: 2110.500
Average: 3.324
Median: 3.500
EDIT: Line 9 specifically accounts for this:
Code:
if fm['datatype'] not in ('int', 'float', 'rating'):