Grouped, cumulative total.M

// Code written by Hans Peter Pfister, www.powerbi-pro.com
let
    Source = Lagerbewegungen,
    Calc_Running_Total = Table.Group(
                                      Source,                                                                  // table as table
                                     {"Produkt"},                                                              // key as any
                                                                                                               // aggregatedColumns as list 
                                        {{"Daten",                                                             // name of new column to be created
                                    // Function Call
                                        (Input as table) as table =>                                           // data for function
                                        let
                                        Call_Function = fn_cumulative_Total(Input),
                                    // End of Function Call

                                        result = Table.FromColumns(                                            // tidy up
                                                                    Table.ToColumns(Input)&{Call_Function},
                                                                        Value.Type(
                                                                                   Table.AddColumn(
                                                                                                    Input, "Saldo", each null, type number
                                                                                                   )
                                                                                )
                                                                       )
                                        in
                                            result,
                                    type table}}                                                                // transform into table
                                    ),

    Expanded_Data = Table.ExpandTableColumn(Calc_Running_Total, "Daten", {"Datum", "Betrag", "Saldo"}, {"Datum",  "Betrag", "Saldo"})
in
    Expanded_Data