So, here is the complement of information I wanted to add following your comments. You can see the evolution of sign ups on a wider space of time, and a comparaison with trafic at the same time.
This is a key element showing how the pop-up we added had a strong impact. To be more specific, the welcome message we decided to present to our new customers though a pop-up had a great impact. As we always say on this blog, pop-ups are a great tool, but results depend on how you use it.
It speaks for itself, I guess this should be a reference case about pop-up performance.