Set height of a scrolled-canvas

Hi! I'm trying to simulate a diffraction grating for my pupils. I have two scrolled-canvas: one for displaying the spectrum as colors, and one for showing its intensity profile. The problem is that I want to make the first one to have a specific height (no matter if the window gets maximized), and the other to get expanded, but all I can get is the two having always the same height. What am I doing wrong? Thank you very much, Luis