VHE SED of Mrk 421 derived from the MAGIC-I observations performed in the eight time slots with tightly simultaneous Swift/XRT data (see Sect. 4). The spectra are shown after correction of the EBL absorption, following Franceschini et al. (2008). For comparison, the spectrum derived from the observation that registered the highest flux (3.6 C.U. above 200 GeV) of the whole campaign, performed on 2008 March 30. For each night we also plot our model (see Sect. 4.1) to guide the eye.