You probably can't quite see how it works yet, I have enough trouble visualising it myself!
The horizontal section is now mounted to the base plate, free to slide up and down on two steel tubes. The only thing holding it down (other than gravity) is the spring from the spring balance.
As the bow is drawn the spring will stretch and the horizontal section will lift slightly, this lift will be linked to the pen to draw the vertical displacement onto the graph.
If you don't follow that, just trust me I'm an engineer! If you look closely at the first pic you'll see the spring has come off it's lower mounting (whoops).