by month; monthly breakdown
Used to categorize or organize data, schedules, or items according to each month. Often appears in compound expressions like 月別データ (monthly data) or 月別カレンダー (monthly calendar).
We tally sales by month.
I compiled the monthly expenses into a graph.
Compound of 月 (つき, 'month') and 別 (べつ, 'classification' or 'by'). The 別 suffix is productive for creating nouns meaning 'by X' or 'classified by X'.