Waterfall Chart

Hello,

I am trying to create a waterfall chart but I could not find a way to make it work in the way that I need. I tried several combinations of categories and breakdowns, tried solutions listed on different posts and video tutorials, but none of them got me to the desired result. 

I have a list of active projects that is updated monthly. On each month we can open new projects and close active ones. I need just a chart that displays the total of each month in categories and on the breakdowns the number of projects that were open and closed between each month, as pictured:

It doesn't sound something too complicated to do, so probably I am missing something obvious. Can anyone help me?

Thanks in advance.