Top row, left: boxplot of the estimated mixing-length values for an artificial offset of −0.1 dex in the observed [Fe/H] values (scenario AS09-Mm) in the adopted metallicity bins. The recovery was performed with a grid computed at ΔY/ΔZ = 1.0. Middle: same as in the left panel, but with a recovery grid computed with ΔY/ΔZ = 2.0. Right: same as in the left panel, but with a recovery grid computed with ΔY/ΔZ = 3.0 Bottom row: same as in the top row, but for an artificial offset of +0.1 dex in the [Fe/H] (AS09-Mp scenario).