Sometimes, somebody wants a different layout concerning your datawindow. Okay, I can respect that. One time, I'd got a question if it was possible to show a line every three rows. Well, at first I thought no, that's not possible but I've figured a way out that's very simple and works like a charm.
For this, add a line to the detail band, as low as possible and stretch it out to the maximum width of your datawindow. Call it whatever you want. RightMouseButton click on it and enter the following expression for the visible attribute:
if ( mod ( getrow(), nn ) = 0, 1, 0 )
where nn is the number of rows between the lines.