I love the concept of this figure, and I like how the right panel is built to present, but the actual results mapped onto the triangle are hard to figure out. Like this is the actual experiment they ran , but its not clear what represents what. Seems like maybe a density map might be more appropriate.
These ternary plots are also commonly used for compositional data, e.g. for displaying a property of a three component mixture. Its three components shall always sum up to 100 %, thus the axes are increasing in opposite directions to each other.
They are common and yet I still really struggle to quickly understand what any points but the three extremes mean. I'm not sure there's an alternative though.
Took me a minute to fully parse, I'll try to explain
Each edge of the triangle is 0-100% of each mode, thus the center is 33% of each mode because of the skew in the 'grid'.
Then the thickness/color represents the population, the data just happens to work that there's a strong correlation between population and change in modal percent, making the constant gradients.
The graphs on the left convey essencially the same information. It would be a lot more interesting to see where individual nations/cities sit within the ternary graph