Cohorts are always a great way to split a group into segments and get a deeper view of what ever you looking at. Imagine you have an online shop and would like to know how your user retention has developed over the last view weeks.
The diagram above basically shows the retention rate of fifteen different groups. For example about 25 percent of the people from cohort one came back to visit our online shop 15 weeks after their first visit. Cohort fifteen visit the online shop for the first time this week that is why we just have data from one week. With this principle in mind you can analyze your retention rates over time.
The code to create this ggplot can be found on my Github.