NLTE corrections for Na I, colour-coded by log g: squares are from Lind et al. (2011), triangles from Alexeeva et al. (2014), and circles from Lind et al. (2022). The rows show three metallicities: [Fe/H] = −2 (top), [Fe/H] = −3 (middle), and [Fe/H] = −4 (bottom). The columns show different [Na/Fe] LTE values: [Na/Fe] = −0.6 (left), [Na/Fe] = 0 (middle) and [Na/Fe] = +0.6 (right). The symbols are hatched diagonally in cases where only one line is available (i.e. EW > 5 mÅ). The error bars represent the standard deviation of the NLTE corrections of the different Na I lines at a given set of stellar parameters.