I am not providing full solution, but here is a hint how to handle previous months.
Dashboard action does not allow to select other month with normal data structure, then need to re-shape the data.
Because smart join is not available at 10.1 yet, Create simple table like below.
Copy this and paste clipboard data..
Then edit data datasource
Now you got this data, but be careful data is duplicated.
Last Month Help_SM_10.1.twbx 114.4 KB
This looks really great! Thanks so much! Just a question: The table that you created, will the Link ID be 1 for every month?