Use a summary field that computes the sub totals that you want to chart. Sort to group the records by month. In your data source options, be sure to specify the option for "found set", "summarized data". (I'm not recalling the exact wording.)
To group your records by month, one way is to add a calculation field with date as the return type with this calculation:
YourOriginalDateField - Day ( YourOriginalDateField ) + 1
This computes a date for the first day of the month for all dates in the same month. You can then specify this field in your sort to get one date point for each month.