Hi @maucejo ,
passing a ratio to diagram.width will be possible starting with lilaq:0.5.0, I implemented this here last week.
The text set rule should just work fine (check it out by adding a diagram.title for instance). Note that Lilaq uses Zero for displaying tick labels on axes, so the math font is actually used here! Either you need to use a math font or tell Zero, not to use math for its number formatting by setting math: false via
#import "@preview/zero:0.4.0": set-num
#set-num(math: false)