Customers of subscription services can upgrade or downgrade across plans. Hence, across months, users are actually moving across plans.
Hover over the "Advanced" (Blue) stack in the month of March and you will see that most of your "Advanced" customers in month of February continued their subscription while a small percentage of "Basic" customers upgraded. However, going from March to April, a huge percetage of customers stopped their service.
This is a modified stacked bar chart with flows across stacks.