He⁺ abundance vs. [O iii]λ5007/Hβ. The first-degree polynomial fit is shown with a black line. The 1-σ and 3-σ levels are marked with thick and thin red lines respectively. Mean and standard deviation of each 0.05 dex bin in log  ([O iii]λ5007/Hβ) are shown with blue diamonds and error bars respectively. The green horizontal dashed line shows the mean value. Data points corresponding to spaxels with log   (N/O) >  − 1.3 have been marked with orange circles.