Power Query Grouped Totals offset

Hi, 
 If I  do a group total by year  so ; 

= Table.Group(Source, {"Year"}, {{"YT", each List.Sum([Unit]), type nullable number}})

 and then aslo want  example the previous year as another column to compare I can just add a column to this ; 

 Table.AddColumn(#"Grouped Rows", "Custom", (A) => 
let result= 
        List.Last(
            Table.SelectRows(#"Grouped Rows", (B) => B[Year] < A[Year])[YT]
        ) in if result = null then "NA" else result) 

But is is possibel to do this all withingthe Grouped Rows aggregartion? 
I have tried adding a column within the group but it's not right

 Table.Group(  StartTable, { "Year"}, { {"ThisYear",each List.Sum([Unit])  } , 
{"All",  each   Table.AddColumn(  StartTable, "N", (A)=> 
List.Sum( 
  Table.SelectRows( StartTable, (B)=> B[Year] < A[Year] )[Unit] ) ) [N]    }} )

so Im trying to end up with somthing like this; 

Any adivice on how to proceed welcome, 

Richard.