Example of line core fit of the 3D profiles of the line Fe Iλ_lab 5379.5740 Å described in Sect. 4. Left panel: line 3D profiles for three line strengths at solar abundance ±0.1 dex (black dots) together with single Gaussian fits (red dashed lines) and the laboratory wavelength (vertical green dashed-dotted line). Right panels: parabolic fits (blue solid line) to the line core of the three 3D profiles, together with single Gaussian fits (red dashed lines). Vertical lines show the wavelength position of the centroids of the parabolic fit (blue dashed-three dotted line) and the Gaussian fit (red dashed line), together with the laboratory wavelength (vertical green dashed-dotted line).