The bold line in the diagram (Hertling & Kessler, 1996, Fig 16-10C, p.499) approximates the axis around which the ribs move at the costovertebral joint.
This line connects two articulations:
The diagram (Kapandji, 1974, Fig.15, p.139) shows the costovertebral axes from above. The different axis orientations produce different arcs of rib motion.
When the more superior ribs elevate, their movement expands the ribcage in an anterior direction.
When the more inferior ribs elevate, their movement expands the ribcage in a lateral direction.
Kapandji, I.A. (1974). Movements of the ribs at the costovertebral joints. In I.A. Kapandji, The physiology of the joints: Vol. 3. The trunk and vertebral column. (2nd ed.). New York: Churchill Livingstone.
A text that contains an advanced but well-organized discussion of ventilatory biomechanics (pp. 178-192) is Norkin, C.C. & Levangie, P.K. (1992). Joint structure and function: A comprehensive analysis. (2nd ed.). Philadelphia: F.A. Davis.