Как правильно настроить ассоциации в ExtJS 5 и загрузить их по требованию?

Кажется, вы считаете месяцы упорядоченным фактором. Вы должны сделать это в R:

Month = c("April", "April", 
          "April", "April", "April", "April", "August", "August", "August", 
          "August", "August", "August", "December", "December", "December", 
          "December", "December", "December", "February", "February", "February", 
          "February", "February", "February", "January", "January", "January", 
          "January", "January", "January")

Month.ordered <- ordered(Month, month.name)
#[1] April    April    April    April    April    April    August   August   August   August   August   August   December December December December
#[17] December December February February February February February February January  January  January  January  January  January 
#Levels: January < February < March < April < May < June < July < August < September < October < November < December