I tried the expression you have given, it gives a cumulative chart. But I'm looking for something slightly different. In other words, for each day, I find out number of new customers, and sum up with that value for the previous day.
Refer to my sample data in my original post. If I were to plot using your expression, the result for Day 2 would be 5. However, my expected result is 3 because there are 2 new customers on Day 1 and only 1 new customer on Day 2.