…how did the line come about? How did they determine what the life expectancy would have been with less expenditure per capita?
My guess is that this was a gif at some point and the line is historical data
Definitely, you can see some lines in the top left zigzagging back left, which would not be possible if each was a function of the x-axis. In fact, both axes are a function of the hidden z-axis, which is time and comes in discrete yearly steps, the latest of which (2021) is highlighted.
At least one of those lines goes back on itself at some point, so my assumption is that it’s tracking where each country has been over time.
There is a minimum amount which is likely the least some people spent on their health. So there is no interpolation I can see.
