change script to
LOAD Date(Date) as Date,
Month(Date) as Month,
(ooxml, embedded labels, table is Sheet1);
Adding the Month Section, then you get:
With Month as the dimension and Sum(Sheets) as the expression
Or if you can't change the script you can create a calculated dimension in your chart, using the expression =month(Date), with label Month and use the same expression.
This will aggregate the sheets by Month
Less efficient that changing the script though, so if you have the option, go for the script change.