Combined 2005 Q-mode data rebinned with a factor of 2. Upper panel: the strength of the 12P modulation pattern as a function of pulse longitude (dotted curve) and the pulse profile (black curve). Lower panel: the corresponding subpulse phase. An arbitrary phase offset has been applied to make the subpulse phase zero at the peak of the profile. The dotted line indicates the gradient of the phase relation expected for a modulation pattern that is longitude stationary in the co-rotating pulsar frame.