Illustration of the MCMC analysis for the spectrum of TW Hya obtained on 2009-02-12 using Model 4. The corner plot at the top shows the posterior distribution of the model parameters (magnetic filling factors f_i and the scaling factor k). The bottom histogram displays the resulting posterior distribution of the magnetic field modulus ⟨B⟩, overplotted with its median value (yellow thick line) and the limits of the 3σ confidence interval (red dashed lines).