It seems that you need a feature that is not present in the chart view - two-level-grouping. Once you need to group the items by year-month-day, then group them by year-month. This is currently not present in this control.
I can suggest that you add the series through code behind and handle the stacks manually. I have attached a demo project to demonstrate this approach and a snapshot of the output it produces. Here I create a new series for each bar and set its proper stack group key and category. Usually I would not suggest such an approach (adding many series with a single item in a series) but I think this is the only way to achieve the scenario in mind.
All the best,
the Telerik team