cumulative line/area chart

Hi all, 

For the life of me I cannot get a line/area chart that would show a cumulative increase, e.g. users bought 10 courses in Jan, 3 in Feb, 0 in Mar, 15 in April, and a line chart would show growth (not drops as per monthly decrease), meaning 10->13->13->28.

The data I have are what I described - calendar and numerical values of purchases per given month.

Thanks a lot in advance!