 ![Si line ratios overlaid onto a NEI grid. Upper panel shows the full range of values for all the regions (region R0 is located off the graph), and the lower panel zooms in on the cluster of values in the upper region of the graph. Temperature, $kT_e$, increases along the lines which go generally from upper-left to lower right. Ionization time, $\tau$, increases along the lines which are nearly horizontal at left and move to the upper-right ending at the high-$\tau$ limiting asymptote. A few $kT$ values are labeled in red, while some $\tau$ values are labeled in blue.[]{data-label="fig-line_ratios"}](f0