Gas fraction plane, a relation between Hi mass fraction and a linear combination of stellar mass surface density and observed NUV − r color. a) Relation obtained using all the Hi detections in the GASS representative sample (red circles). The symbols are the same as those in Fig. 5. b) Relation obtained using only the subset of detected galaxies with NUV − r  ≤ 4.5 mag (red circles). Gray circles and green upside-down triangles indicate the remaining Hi detections and the non-detections, respectively.