Different strong-line EWs against Hα EW after zero point correction. The blue cross denotes the origin. The grayscale represents the density of the spaxels, while the orange contours enclose 10%, 68%, 95%, and 99% of the spaxels in each panel. The 10% contour is used to show that most of the data are centered at the zero value. The line in the first subplots represents the cut we applied to exclude contamination from transition objects and dusty star-forming galaxies. All spaxels on the right side of the line are removed from the next step.