[NII]- and [SII]-BPT diagrams with each spaxel colour-coded based on its spatial location: inside an HII region (purple circles), a region ionised by another source (grey stars) or in the DIG (green diamonds). Observations are compared with fast radiative shock models from Allen et al. (2008) (shock only models for v_shock <  200 in red; shock+precursor models for v_shock >  200 in dark blue) and models of star forming galaxies from Levesque et al. (2010) (solid black). The black dashed line shows for reference the extreme starburst line from Kewley et al. (2001).