This might depend on how many different cost codes you have, how often they change, and how many months you want to go back on.
If all three of those are limited, you could do it with specific summary fields for each, but if you need it to be more dynamic, you will need to create a "Cost Code Summary" table.
Each record will be a combo of month+cost code.
How you create those records will need to be through a custom script.
I wish it was more simple, but when you are summarizing on multiple data points (date & code) you have to have a record that ties those together. At least for graph reports.
With your current set up you could create a few summary reports that might give you close to what you need.
