What is the x-axis in that Age of the Doctor graph? Is it time, or is it Doctor index (i.e., the value of n in the expression “nth Doctor”)?

I ask because the relationship of the last three Doctor ages on the graph looks remarkably linear to me. If we’re willing to be irresponsible and abusive with statistical concepts (and I certainly am), we could try to derive a linear equation from this and use it to predict the ages of future Doctors — who will, of course, continue to get younger until the role is being played by an infant. (I’m assuming that the Doctor will find some way around the standard limit of twelve regenerations.)

This isn’t unprecedented. If you examine the graph, you’ll see that in the period comprising the third, fourth, and fifth Doctors, the age graph was also linear, and in a steeper decline than it is now. Fortunately, this trend did not continue.