If spectral is FALSE (the default), produces a threshold
choice plot as illustrated in Beirlant et al. (2004). With \(n\)
non-missing bivariate observations, the integers
\(k = 1,\dots,n-1\) are plotted against the values
\((k/n)r_{(n-k)}\), where \(r_{(n-k)}\)
is the \((n-k)\)th order statistic of the sum of the margins
following empirical transformation to standard Frechet.
A vertical line is drawn at k0, where k0 is the largest
integer for which the y-axis is above the value two. If spectral
is FALSE, the largest k0 data points are used to plot an
estimate of the spectal measure \(H([0, w])\) versus \(w\).