Replace null with average value of above/below value

marfor 

Here is one possibility. But might be slow.

Please see attached file as well.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("VY5JCgAxCAS/Ip49jEsgeUvw/98Ys8g4YF+q6cI5EUzJRgMk5EiLoFNy27wvSCilUDhsTz4sF/NfwwubXD+X5oF010Ef+U+covsL", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [#"DATE:" = _t, MG = _t, TG = _t, OW = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"DATE:", Int64.Type}, {"MG", Int64.Type}, {"TG", Int64.Type}, {"OW", Int64.Type}}),
    #"Changed Type1" = Table.TransformColumnTypes(#"Changed Type",{{"DATE:", type date}}),
    #"Renamed Columns" = Table.RenameColumns(#"Changed Type1",{{"DATE:", "DATE"}}),
    myfunction=(PrevStep as table,ColName as text,mydate)=>
        let 
        mytable=Table.SelectColumns(PrevStep,{"DATE",ColName})
            in
        let 
        RA=Record.Field(Table.Max(Table.SelectRows(mytable,(x)=> x[DATE] < mydate and (Record.Field(x,ColName)<>null)),"DATE"),ColName),
        RB=Record.Field(Table.Min(Table.SelectRows(mytable,(x)=> x[DATE] > mydate and (Record.Field(x,ColName)<>null)),"DATE"),ColName)
        in
        List.Average(
        List.RemoveNulls(
         List.Transform(
        List.Positions( {RA,RB} ),
        each try {RA,RB}{_} otherwise null
        )
        )),

    Replace1 = Table.ReplaceValue(#"Renamed Columns",null,each myfunction(#"Renamed Columns","MG",[DATE]),Replacer.ReplaceValue,{"MG"}),
    Replace2 = Table.ReplaceValue(Replace1,null,each myfunction(Replace1,"TG",[DATE]),Replacer.ReplaceValue,{"TG"}),
    Replace3 = Table.ReplaceValue(Replace2,null,each myfunction(Replace1,"OW",[DATE]),Replacer.ReplaceValue,{"OW"})
in
    Replace3