How Do I Vary an Endpoint of a Line in an Array?

Ok, I am an absolute beginner at Grasshopper.  I have absolutely no idea what I am doing so I apologize if I don't use the correct terminology.

I have lots of line geometry contained within a square polyline curve that exists in Rhino.  I have three separate lines in the y direction; a centerline, a right, and a left line.  

The lines are fixed where they intersect the polyline at the furthermost edge in the y direction.  The right and left curves are parallel to the center curve whose unfixed point varies between .25 and 2.75.

I want to know how to create an array of 50 of these polyline groups so that the center line varies randomly within its range preferably at stepped intervals of .25.

I understand how to create arrays, I just need to understand how to create specific variation within that array.  I have attached the gh file and screenshots of the polyline geometry and the grasshopper data.

Any help would be greatly appreciated!

GrasshopperHelp.gh