The phase diagram gives one point per year for the period 1844–2011. The left-hand diagram is for the standard data; the other is for the Lockwood data. Data prior to 1968 are from Nevanlinna & Kataja (1993) in the two panels. From 1968 onward the aa_min data are as given in Figure 2. The abscissa and ordinate of the horizontal and vertical lines are the coordinates of the Transition Point. There is a difference between the value of the ordinate of the Transition Point for the standard data (left) as compared with that derived with the Lockwood data.