Multi-epoch evolution of NGC 4151 X-ray parameters compared to the He I column density, along with the corresponding 1σ uncertainties. Top panel: soft X-ray flux $F_{\mathrm{X}}^{(\mathrm{s})}$ (0.5–2 keV; red points). Upper mid panel: Column density $N_{\mathrm{H}}^{(\mathrm{X})}$ of the X-ray neutral absorber (green crosses). Lower mid panel: hardness ratio of the X-ray spectrum (blue squares). Bottom panel: column density $N_{\mathrm{HeI}}^{(\mathrm{tot})}(2^3\mathrm{S})$ of the He I absorber (black stars). In the column density panels, the zero level (dotted line) is indicated.