I would suggest you to create a new field in your data-model like
MonthName(Date_Field) as MonthName
Then use this field in your chart as dimension, this will be properly sorted.
=Year * 100 + Month
By this you will always have numbers that are 6 digits
ex. 201401, 201402,201303,..... and so on
try with sort by this expression (in sort tab)
=Year & num(Month, '00')