Residual metallicity vs. metallicity gradient. Each circle presents one single galaxy in our sample, and the red shaded region represents 1σ uncertainties of the best-fit relation; the slope is shown as a red dashed line. In both panels the X-axis represents the metallicity residual of the inner region or outer region of the galaxy in the left or right panel, respectively. The results of best linear fit are shown in the upper right corner in each panel.