The DEW diagram plots Dn(4000) against max(EW[OII],EW[NeIII]) and was developed by Stasinska et al. (2006).
The dividing line is:
Dn_4000 = -0.15 *(log10(x) + 1) + 1.7
where x = max(EW[OII],EW[NeIII]) (and where EW[OII] is the equivalent width of the [OII]3727 doublet, and EW[NeIII] the equivalent width of the [NeIII]3869 line). AGN tend to reside above this line with elevated Dn(4000) relative to purely star-forming galaxies.