Need help using graph to control Z values!

Hey everyone!

I have a small dilemma here. I'm trying to control my z values using a self-made graph from intersection points made by a curve in rhino as you'll see in the second image.

I'm trying to control the height of my balustrades with the graph curve on the top view but it keeps slanting. I'm assuming that I'm plugging this in all wrong? I'm trying to get my height of my balustrades to fluctuate smoothly along the curve (kinda like a wave, I guess).

It would be great if someone could have a look at my definition or could tell what's going wrong with my definition. If anyone has suggestions on how to fix this or knows a better way of doing this, feel free to comment!

Grasshopper file and 3dm file are attached below if anyone wants to have a look.

Thanks in advance!

Ra