Échelle diagrams of the radial modes of a typical red-clump giant, comparing the asymptotic expansion (Eq. (6), blue triangles) and the development describing the curvature (Eq. (9), red diamonds). Top: diagram based on Δν_obs observed at ν_max; the dashed line indicates the vertical asymptotic line at ν_max; the dot-dashed line indicates the asymptotic line at high frequency. For clarity, the ridge has been duplicated modulo Δν_obs. Bottom: diagram based on Δν_as; the dot-dashed line indicates the vertical asymptotic line at high frequency.