Profile evolution for three MSPs within the LOFAR HBA band. The number on the left in each panel gives the central frequency of the corresponding sub-band in MHz. The profile in grey on top of each panel is the average profile within the 78-MHz wide band. The profiles for each sub-band are normalised by its noise rms deviation. Each sub-band profile is overlaid with the average profile of the total band, in grey. To keep the integrated flux densities the same, the overlaid total-band average profile (in grey) is normalised by the ratio of areas under the total-band profile and the sub-band profile.