Remove null values to make sorting works accordingly

You could correct the nulls by using the following beastmode:

IFNULL(`YourValue`, 0)

You can also sort any values using a beastmode like this:

CASE 
WHEN `Transaction Month` = 'Jan' THEN 1 
WHEN `Transaction Month` = 'Feb' THEN 2 
WHEN `Transaction Month` = 'Mar' THEN 3
WHEN `Transaction Month` = 'Apr' THEN 4 
WHEN `Transaction Month` = 'May' THEN 5 
WHEN `Transaction Month` = 'Jun' THEN 6 
WHEN `Transaction Month` = 'Jul' THEN 7 
WHEN `Transaction Month` = 'Aug' THEN 8 
WHEN `Transaction Month` = 'Sep' THEN 9 
WHEN `Transaction Month` = 'Oct' THEN 10 
WHEN `Transaction Month` = 'Nov' THEN 11
WHEN `Transaction Month` = 'Dec' THEN 12
Else 13
END