Dates improperly aggregating in visualization

A few things you need to fix.

1.  My example of using a number (1-12) was an example if you have a Month column.  Since you are using Month and Year, you need to make a unique order column for this value.  I would make a calc column (query editor) that takes Year*100+Month.  This will give you values like 201601 for Jan2016, 201602 for Feb2016 etc.  Make sure the data type is numeric and this will allow you to sort correctly across multiple years.

2.  You don't need to concantenate this unique sort order column with the Month Year column.  Just select the field from the fields list, go to the Modeling tab, and choose Sort By > Unique Sort Order column.