ABSTRACT Using the MEGAFLOW survey, which consists of a combination of MUSE and UVES observations of 22 quasar fields selected to contain strong Mg ii absorbers, we measure the covering fractions of C iv and Mg ii as a function of impact parameter b using a novel Bayesian logistic regression method on unbinned data, appropriate for small samples. We also analyse how the C iv and Mg ii covering fractions evolve with redshift. In the MUSE data, we found 215 $z = 1{-}1.5$ O ii emitters with fluxes ${\gt}10^{-17}$ erg s−1 cm−2 and within 250 kpc of quasar sight-lines. Over this redshift path $z = 1{-}1.5$, we have 19 (32) C iv (Mg ii) absorption systems with rest-frame equivalent width (REW) $W_r\gt $ 0.05 Å associated with at least one O ii emitter. The covering fractions of $z\approx 1.2$ C iv (Mg ii) absorbers with mean $W_r\approx$ 0.7 Å (1.0 Å), exceeds 50 per cent within 23$^{+62}_{-16}$ (46$^{+{18}}_{-13}$) kpc. Together with published studies, our results suggest that the covering fraction of C iv (Mg ii) becomes larger (smaller) with time, respectively. For absorption systems that have C iv but not Mg ii, we find in 73 per cent of the cases no O ii counterpart. This may indicate that C iv comes from the intergalactic medium (IGM), i.e. beyond 250 kpc, or that it is associated with lower mass or quiescent galaxies.