Since this is an iterative function, you are not looking at a value y that is a result of a function of x. But like they said in the article, it is more representative of time-series data where we are only looking at the progression of x. However each value of x at an iteration i is not a function of i, but is based on the value of the x that came before it (x(x-1). So even though they did not label the "x-axis", it is really just a sequential progression of calculating the next value of x. So really, the "y-axis" is really the value of x calculated, and the "x-axis" is just the next iteration of calculating x.