Plot an expression along a specific coordinate system

Hi

I defined a cylindrical system (r,phi,a) and I'd like to get the results along one of these specific coordinates.

Since all the direct results expressions are defined in cartesian coordinates (x,y,z). I would like my magnetic field along x "mf.Hx", as an example, to be expressed and shown along r.

I found that the way to do so seem to insert an expression in the plot expression setting. I found that the cylindrical system is included in the definitions window. But I can't find a way to have my expression calculated along r since I get an error. Also, I can't figure out which one to select from the list between "Base vector (sys2) a", "coordinate, a component (sys2.a)" or "coordinate transform to system (sys2)".

What should I do please ?

Thanks a lot