Relative aerodynamic drag. Aerodynamic drag is on the vertical axis, in multiples of a true aero shape. Each shape has the same frontal area, so the chart compares the drag due to shape only.
Squoval 3 tubes aren't just stiff and light, they're also incredibly aerodynamic.
For many years now, Cervelo used the above chart with the green shapes as a reference. Now in red, they've added a path showing an evolution from square, to radiused, to convex sides leading to the surprisingly low drag of Squoval 3.