Left: spatial frequencies (u,v) coverage for each dataset. Right: visibility amplitudes (with their respective error bars) as a function of spatial frequency (B/λ in units of rad^-1) for each dataset. Epochs 2011, 2012, 2013, and 2014 are represented from top to bottom. Colors indicate position angle (PA) ranges. Black circles correspond to regions around polar axis of the star PA_pol = 36.9° (14.4° ≤ PA ≤ 59.4°), hollow circles match the equatorial axis PA_eq = 126.9° (104.4° ≤ PA ≤ 159.4°), and red circles correspond to the other directions.