Synthetic stretched period échelle diagram built from Eqs. (9) and (10). The colours indicate the azimuthal order: the m = {−1, 0, +1} rotational components are given by green triangles, blue dots, and red crosses, respectively. The small black dots represent mod Δ Π₁. The numbers mark the crossing order k (Eq. (18)). As the number of observable modes is limited, they cover only a limited frequency range in the diagram.