let
    src = Excel.CurrentWorkbook(){[Name="Таблица1"]}[Content],
    tp = Table.TransformColumnTypes(src,{{"Месяц", type date}, {"Значение", type number}}),
    srt = Table.Sort(tp,{{"Месяц", Order.Ascending}}),
    ind = Table.AddIndexColumn(srt, "Ind", 0, 1, Int64.Type),
    add = Table.AddColumn(ind, "Gen", 
                              (x)=> List.Generate(()=>
                                    [y = 0, v = ind[Значение]{y}, s = {v}],
                                    each [y] < Table.RowCount( ind),
                                    each [y = [y]+1, v = if ind[Значение]{y} = null then List.Average( List.LastN([s], 12)) else ind[Значение]{y}, s = [s] & {v}],
                                    each List.Last ([s]) 
                                    ){x[Ind]}
                            ),
    rmv = Table.RemoveColumns(add,{"Значение", "Ind"})
in
    rmv