Diagnostic diagram from Rose et al. (2018) to simultaneously measure the reddening E(B − V) and n_e of the ionized gas. The open symbols with error bars correspond to our flux measurements of the narrow (n), intermediate (i), total (t) and broad (b) components (circle, down- and up-pointing triangles, and plus symbol, respectively). Colour symbols correspond to the grid of photoionized models computed with Cloudy.