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.
My guess is that the line tracks what the life expectancy was when the expenditure per capita was that much? Might have to dig into their source to get more details.