DataWindow Group By N Horizontal Lines Every Rows
DataWindow Group By N Create a group for the DW that groups by the expression Long((GetRow()-1)/N) where N is the number you want to define your group by. Put the sum in the group footer as usual.
DataWindow horizontal lines every rows Add a line to the detail band, as low as possible; stretch it to the maximum width of your datawindow. Righ click on it and enter the following expression for the visible attribute where N is the number of rows between the lines.
if ( mod ( getrow(), N ) = 0, 1, 0 )