I am a new user to QlikSense, so may have this totally wrong or be oversimplifying, but I recently posted a similar question for bar charts and discovered two things that helped me get months shorted chronologically:
1. Don't load month and year data separately from your data source. Load a single date that includes month and year and order by this dimension (by dragging it to the top of your list in the 'data' menu of QlikSense).
2. If you are loading data from Excel, you will need to translate Excel's serial number format into something usable in QlikSense. I used the formula date#(date(InvDate, 'MMM-YYYY'), 'MMM-YYYY') as Date which gave me my loaded dates but with just the Month and Year (no Day) and the correct ordering. InvDate is the name of my Excel data field that contains the date I wanted.