link pairs of rect by vertical line

[xedi...@gmail.com]

I need to show the vertical lines to link the pair of rects. By pair of rects, I mean to say the Branch 4.1, Branch 4.2 part. How do i show it? I tried using d.source and d.target for it but did not work. Here is what I could do where the lines to show, i have no idea on it. Can anyone help me at this, please?

here is the source code in jsbin

http://jsbin.com/qiyejeligi/edit?html,css,js,output