Spread and evolution of the (log) Fe I column density among the 12 excited levels over time. The black circles refer to the column density of each excited level relative to its median value as observed from 2003 to 2017. The grey triangles are upper limits derived from weaker absorptions. A few points outside the plot window are shown with arrows next to the axis. The solid black line shows how the total column density of excited Fe I varies over time. The cyan and green solid lines show the evolution of the column densities derived, respectively, for the Fe I₄₁₆ and Fe I₆₉₂₈ excited levels. The Fe I₀ variations derived in Table 4 are shown as a solid red line.