Stacked residuals of Mrk 421 RGS1 and RGS2 spectra as described in the text. Here x represents the wavelength in Å, and y the data/model ratio. The red curves show a simple analytical model for these residuals, with shape and parameters shown in red in the insets. Negative outliers are associated with the hot or bad pixels that are not perfectly taken into account in the simplified stacking procedure.