Dust-to-(total) gas mass ratio scaling relations with stellar mass surface density (Σ_★, top) and gas metallicity (bottom) in M 99. Filled circles show THEMIS dust masses, colour-coded by molecular gas fraction (f_mol); grey hexagons indicate single-T MBB dust masses. Each point represents an 8″ ∼ 560 pc pixel. The angular resolution is 25″ ∼ 1.75 kpc. The bottom panel covers only regions with metallicity measurements (Fig. B.5). Solid red and dashed black lines show best-fit relations for THEMIS and MBB, respectively; green lines show correlations from Casasola et al. (2022). Red and white diamonds mark integrated values for THEMIS and MBB fits. Calibration uncertainties are indicated by grey bars near the legend.