Projection of a small part of a spectrum into the (df₀∕dλ, f, δf) space that we call the shell space. Top: reference spectrum used to perform the change of variables (in black). The coloured dots correspond to the value of δf when comparing a Doppler shifted spectrum to the reference spectrum. Bottom: projection of the Doppler shifted spectrum onto the shell space. Each point in this new space corresponds to an original point of the reference spectrum and the colour coding corresponds, as above, to the value of δf when comparing a Doppler shifted spectrum to the reference spectrum. Positions of the lines’ cores were indexed to make the identification easier. The mean velocity difference between the shifted and reference spectra δv can be recovered as the slope between the flux gradient and δf.