let
    Source = Excel.CurrentWorkbook(){[Name="Данные"]}[Content],
    group = Table.Group(Source, {"Контрагент"}, {"temp", (sub) =>
    let
        order = Table.Sort(sub, {"Месяц"}),
        base = List.Buffer(order[Значение]),
        count = List.Count(base),
        calc = List.Buffer(List.Generate(
            () => [i = 0, sum = 0],
            each [i] <= count,
            each [i = [i] + 1, sum = [sum] + base{i - 1}],
            each [sum]
        )),
        append = Table.AddColumn(order, "Скользящее среднее", each
            if [Месяц] > 3 then (calc{[Месяц] - 1} - calc{[Месяц] - 4}) / 3 else null
        )
    in
        append
    }),
    result = Table.ExpandTableColumn(group, "temp", {"Месяц", "Значение", "Скользящее среднее"})
in
    result