Scale NU scale both ways on one axis

Hi, I am new to GH and I am following along a tutorial I found online. I did exactly what the tutorial told me to do but there is a step that my GH file that did not react the same way as the tutorial. Hope to seek an answer here.

As I high lighted here. When I connect a scale slider to Scale NU, my green extruded plane only scales inwards. (from the h frame) I need it to scale both ways inwards and outwards on X-axis. What should I do?

Thank you

Oval to Dome.gh (23.5 KB)

Files?

Sorry. Attached.