log([N II]/Hα) vs. k>g([O III]/Hβ) diagram of the nebulae before (left) and after (right) the correction for DIG contamination. Only nebulae where Hα, Hβ, [O III]λ5007 and [N II]λ6584 are detected at 3σ after the DIG correction are plotted in the right panel. The colour map qualitatively shows how the points are concentrated. The solid and dashed black lines are the relation from Kewley et al. (2001) and Kauffmann et al. (2003) which separate star-forming regions from regions ionised by other ionising sources.