What is pL scale?

For a ligand L, $\mathrm{pL} = -\log[\mathrm{L}]$, where $[\mathrm{L}]$ is the concentration of the free ligand. A pL scale is an axis of pL on which the domains of predominance of the central ion and of its complexes are drawn, as the domains of acids and bases are drawn on a pH axis.