I make the same mistake all the time for some reason, though I know which is which. I have a theory the reason is that the X axis is often used to plot years (Y), which messes with my brain ever so slightly.

That said, I don't think the Y axis should necessarily start in zero in a graph that seeks to show the pattern of growth rather than the number of users in absolute terms. If anything, a longer X axis would have been more useful, in order to show how unusual such a growth pattern is.