ΔF_rise/ΔF_decay vs. Δt_rise/Δt_decay ratios for all the optical flares identified in this work. Dashed lines mark where these properties are symmetrical. Colour-coding according to the optical/X-ray lags (see Sect. 3.2): zero-lagged flares (green points), positive-lagged flares (red triangles), negative-lagged flares (blue stars), frequency-dependent lagged flares (inverted purple triangles), and uncertain (grey squares). Flare numbers of highly asymmetric flares with longer time decays are labelled.