> and if they used circles of different sizes, it would be easy to mistake the radii as the measured quantity instead of the area
No, if the measured quantity is represented as the radius, everyone will assume it's the area, and you've designed a very bad graph. If the measured quantity is represented as the area, everyone will assume it's the area, and you're fine. The area is what you can see.
No, if the measured quantity is represented as the radius, everyone will assume it's the area, and you've designed a very bad graph. If the measured quantity is represented as the area, everyone will assume it's the area, and you're fine. The area is what you can see.