This is the starting point, we have (string) labels of the length of an employment. PowerPivot sorts them in this order, from 0 - 1 - 2 to 5.
What we would like to have is this:
This can be achieved by creating a Set for the Row Labels. You can find the option on the (PivotTable Tools) Options ribbon, under "Fields, Items & Sets".
After selecting "Create Set Based on Row Items..." you can sort the items manually and give the set a relevant name.
After creating the set it also appears in the PowerPivot Field List under Sets: