On the horizontal axis are levels of per capita income. Figure 6.7 depicts the basic ideas.
The x-axis shows the level of income per capita.
The y-axis shows two rates—of population growth and of total in- come growth. Per capita income growth is, by definition, the difference be- tween income growth and population growth—hence the vertical difference between these two curves. Thus, as you saw in Chapter 3 in our discussion of the Harrod-Domar (or AK) model, whenever the rate of total income growth is greater than the rate of population growth, income per capita is rising; this corresponds to moving to the right along the x-axis.