Goodness of the proxy at Doppler velocity = 37 km/s. Panel (a) gives a spatial distribution of (in)correctly identified pixels. The pixels are colour-coded to show true positives (TPs: red), true negatives (TNs: white), false positives (FPs: green) and false negatives (FNs: blue). Panel (b) represents the percentage of (in)correctly identified pixels as a function of the Doppler velocity (v_D) in the blue wing of the line. The dashed black line marks the position in the line wing (v_D = 37 km/s) at which panel (a) is obtained. A scan through the blue wing of the line exhibiting the goodness of the proxy is available online.