in general your data has to have all the rows in that support the groups you want to see (e.g. data set has to have at least 1 row per month). However you could concievable mimic your 'month row' as 12 month footers taht are conditionally suppressed based on todays date (this month).
That being said, it sounds like you don't need any sub reports and you can do what you want as a crosstab in the main report.
column using date field set to year
row using a formula field convering date field to month.