= Solution
The dashed curves are contours of constant <Cook's distance>, labelled 0.5 and 1, in the plot of standardized residual against <regression leverage>. Writing $h_{ii}$ for the diagonal of the <hat matrix>, $s^2$ for the residual <variance> estimate, and $p=8$ for the number of fitted coefficients,
$$
r_i=\frac{e_i}{s\sqrt{1-h_{ii}}},\qquad
\boxed{D_i=\frac{r_i^2}{p}\frac{h_{ii}}{1-h_{ii}}}.
$$
Thus a contour $D_i=d$ has $r_i=\pm\sqrt{dp(1-h_{ii})/h_{ii}}$. On the actual PDF plot, observation 579 has leverage about 0.084 and standardized residual about $-8$, lying beyond the $D=0.5$ contour and below $D=1$ in influence magnitude, approximately $D\simeq0.7$ to $0.8$ from the plotted coordinates. It is unusually light relative to its fitted log-weight and potentially influential because deleting it can materially change fitted coefficients. It warrants checking, not automatic deletion. The solid smooth red curve is distinct from the dashed <Cook's distance> contours.
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