What is the meaning of "rolling" or "rolling rate" here?
This chart shows a rolling 12-month rate of installs by country/region.
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To put it in layman's terms, as it is updated with more more current data, an equal time period "rolls off" the report.
For example, in a 12-month rolling report, the column or report for January 2020 would have information for Feb 2019 - Jan 2020. The column for Feb 2020 would have Mar 2019 - Feb 2020
Here Rolling is used in the context of a rolling rate or a rolling average. This is an averaged calculated for a period that is updated on a regular basis. The average itself is over just 12 months. As a new month's data comes the numbers from the oldest month are removed.
From bizfluent.com A regular 12-month average reduces a year of monthly figures into a single average number. A 12-month rolling average, or moving average, is simply a series of 12-month averages over multiple consecutive 12-month periods. This statistical tool can help you gauge the overall direction of a series of monthly data, because it smooths out the effects of month-to-month changes.