Observed frequency distribution in echelle diagrams. Various symbols represent frequencies extracted from different data sets (see the legend for details). The right-hand side panel shows the echelle diagram for a period spacing ΔΠ_ℓ = 1 = 267.93 s. The left-hand side panel is the echelle diagram for ΔΠ_ℓ = 2 = 153.20 s. Red dots are the frequencies matching the ℓ = 1 period spacing relation, while blue dots are the frequencies matching the ℓ = 2 period spacing relation. It can be seen that red and blue dots present a clear structure in the right and left echelle diagrams, respectively.