Comparison between observed and modeled fluxes for the ionized gas model M_I,opt of I Zw 18. Plotted values in red are the difference between observations and models normalized by the model value. MULTIGRIS produces this plot in cartesian and polar coordinates, we show here the latter for illustration and because it is better adapted to a large number of observations. The black circle, black dots, and orthogonal black lines indicate the model and the red points and red error bars indicate the observations. The color of the sector is green if observations and models agree within errors and yellow if the model agrees within the observed error bar.