Step1:

Drag month in Row shelf.

Step2:

Drag sales into text.

Step3:

Create calculation as

if first()==0 then total(SUM([Sales])) else

lookup(total(SUM([Sales]))-RUNNING_SUM(SUM([Sales])),-1) end

Step4:

Double click this new calculation.

Step5:

Change table calc, compute using order date

Now we can get bottom to top running sum.