Trying to iterate through a HashMap for a report and getting weird outputs

Hello, I’ve been trying to design an activity report separated by year and months inside those years, for which I created a HashMap that sorts these dates correctly. However when I pass this Map as a parameter for the report, through Groovy.
I try to iterate by keys with the usual Band?keys on the template editor, but as an output I get the list of methods for (I believe it is) the Map.

Like this:

Therefore I have 2 questions. The first one is how to solve this problem, I tried searching around Freemarker forums before posting here and they suggested the method .keySet(), which I tried and did not work.
The second one is, is there a cleaner way I can do these classifications of year and month directly on the report bands? Or an alternative solution so that I can end up dealing with lists which I’m more familiar with on CUBA reporting.