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.
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.