Troubled with attractor

I am trying to make an attractor point on grasshopper. The problem is I'm not sure how. I tried some tutorial, but it's not what I wanted. What's shown on tuts are flat horizontal attractor, while mine is mid-air and the points which got attracted do not stay on the flat ground.

This image shows a little description about my intention.

Yellow : Imaginary distance

Red : A line made from Attractor + the Green dots (limited to Yellow area)

So, as I move the long evaluation slider, this Yellow line will move to the north following the little thin red line which stay on the ground.

Any Ideas?