Our idea: fit lines with the simplest model that can do the job, and use one that, while based in physics, is general in the sense that any number of physical models can be tested or constrained based on the model fits.
From Owocki & Cohen (2001): spherically symmetric, two-fluid (hot plasma is interspersed in the cold, x-ray absorbing bulk wind); beta velocity law.  
Visualizations of the wind use hue to indicate line-of-sight velocity and saturation to indicate emissivity; corresponding profiles are plotted vs. scaled velocity where x = -1,1 correspond to the terminal velocity.