How can I make the linewidth of a lines plot be in data space, so that ot changes when scrolling?
There’s no way to do that. You can plot a poly instead though, it might not be completely trivial to compute it given your data points.
You could compute that using buffer from GeometryOps or LibGEOS.
text has a markerspace attribute for this. Can I not use some transformation used for that?
I guess one could use the linewidth attribute attached to an Observable which scales with your data x or y range which you can get from ax.finallimits when you use the built-in rectangle zoom or mouse scroll.
This should be a backend feature. I think it’s not too hard to add, but would need someone to work on it for a couple of days.