Nitrogen enhancement factors of the stars, defined as nitrogen abundance over carbon abundance (x-axis) and oxygen abundance (y-axis) with respect to their initial values (see Table 2). The error bars show the original Δlog(N/C)/(N/C)_i and Δlog(N/C)/(N/C)_i before adopting the maximum of 0.4 dex, or the original values as the uncertainties. Each data point is color-coded based on the reference from which the surface carbon, nitrogen, and oxygen abundance are obtained. The black dotted lines show the baselines of no nitrogen enhancement.