This is a very theorical question and appear more related to "how calculate the user retention" than "how can I create this viz".
If you are asking the first question I think that to calculate a valuable user retention you should use also the number of accesses of that user and not only the last access date.
With this you can have a good KPI in my opinion.
If the you are asking for the secondo one, tell us what do you whant to show and we will help you!!
Thanks Emanuele for you reply.
The data that I have only consist of the user id, the date they joined and the last active session. I have to develop different visualization but the main problem is calculating the retention rate using this data.