With Cumulative formatting, you can get an overview of certain items over time.
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Best described on an example, here is a number of closed Tasks for the past year grouped by months:

The count column can be formatted as "Cumulative" to show progress month-over-month. Simply click on the Count column and select Formatting options.

To better illustrate this, the Show chart option has been enabled:

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Now, by enabling Cumulative, you will see each month added to the previous one.
E.g. in April, you will see all the tasks closed in January, February, March, and April as a single, cumulative value:

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