Four-panel formation diagram of the pixel marked in red in the bottom row of Fig. 10. Top left: Opacity χ_ν divided by optical depth τ_ν. The red curve is the height optical depth is unity, the white vertical line marks Δν = 0 and the white curve is the vertical velocity as a function of height. Top right: total source function. The green curve shows the line source function, the orange curve the temperature. Bottom left: τ_νexp^−τ_ν, which peaks at τ_ν = 1. Bottom right: the contribution function χ_νS_νexp^−τ_ν. The white curve shows the emergent line profile in temperature units, with a scale on the right.