Diagnostic plots for NGC 6811. Panel a: apparent Gaia CMD colored according to the rotation period (Van-Lane et al. 2025). Panel b: distribution of P_rotation and Gaia G magnitude colored according to the temperature. If the temperature was unavailable, the stars are shown as gray dots. Panel c: distribution of P_rotation and temperature colored according to the G magnitude. The BL candidates (black squares) and the automatically detected outliers (red crosses) are highlighted.