Illustration of the continuum-fitting procedure for a spectrum of pure Gaussian noise. Top panel: input spectrum with S/N  =  50. Centre: input spectrum plotted in order of increasing pixel flux (the “pixel ranking” spectrum). The blue line is a fit to the linear regime, with pixels selected as part of that regime ( ± 1σ of the mean) in red. Bottom: input spectrum, with the pixels selected for continuum fitting indicated by red dots. The resulting continuum fit is shown by the solid blue line.