Please find the image for your query. You have to select pivot table in the object selection and frist dim as Customer
Second dim as month
and in expersion you to select sum(amt)
com back to to presentaion tab (find the screen shot for the same)
select Month Diminsion and check the show patila sums check box.
and on the GUI select the mon the drag upward the blue arrow show in vertical way and leave in
Hope you got your answer