Carbon-to-iron (left panel) and carbon-to-oxygen (right panel) abundance ratios as functions of [Fe/H] and [O/H], respectively, in the solar vicinity. The solid lines show the predictions of model MWG-11 by Romano et al. (2019); the theoretical uncertainties arising from stellar nucleosynthesis are highlighted by the pale green areas. The dashed lines show the predictions of model MWG-11 revised following Spitoni et al. (2019, see text and Table 1). The symbols represent 3D non-LTE abundance estimates from high-resolution spectra of thin-disc, thick-disc, and high-α halo stars (dark grey filled circles, light grey filled circles and empty circles, respectively; Amarsi et al. 2019); in most cases, the observational error lies within the symbol size. All abundance ratios were normalised to the solar photospheric composition by Asplund et al. (2009).