The first graph shows smoothed metallicity vs metallicity and each color corresponds to an element. The dispersion of each element should be smaller on the smoothed metallicity axis than in the metallicity one. The black dashed line represents the x=y line.
In the second graph, the transition between a high metallicity and a low metallicity is shown. The red dots are the particles smoothed metallicity and the blue line the theoretical solution (along with the blue area for the standard deviation of the [not smoothed] metallicity).