Diagnostic diagram for the inner 5′′ of NGC 7252. Each point corresponds to a pixel, coloured by radius from the centre of the star forming disc. We highlight the systematic trend of the changing-line ratios with decreasing radius by the curved black arrow. Due to low S/N, we are forced to co-add the outer SW and NE gas streams, and determine an upper limit on N I. For comparison we highlight model grids based on AGN photoionisation (red, Groves et al. 2004), shock ionisation (green, Allen et al. 2008), starburst photoionisation (blue, Levesque et al. 2010) for an instantaneous burst with an age of 1 Myr.