GDP is an absolute quantity. If GDP doubles, then that means something. So readers should be thinking about the distance between the curve and the x-axis.
But 1980 is arbitrary. When comparing 2020 to 2000, all that matters is that they’re 20 years apart. No one cares that “2020 is twice as far from 1980 as 2000” because time did not start in 1980.
This is the difference between a ratio scale and a cardinal scale. In a cardinal scale, the distance between points is meaningful, e.g., “The gap between 1 and 2 is twice as big as the gap between 2 and 4.” In a ratio scale, there is a well-defined zero point, which means the ratios of points are also meaningful, e.g., “4 is twice as large as 2.”
This is the difference between a ratio scale and a cardinal scale. In a cardinal scale, the distance between points is meaningful, e.g., “The gap between 1 and 2 is twice as big as the gap between 2 and 4.” In a ratio scale, there is a well-defined zero point, which means the ratios of points are also meaningful, e.g., “4 is twice as large as 2.”