The plots are:
One dimensional optical depth plots (diagonal);
Two dimensional optical depth plots (lower triangle);
Two dimensional minimum implausibility plots (upper triangle).
The optical depth is calculated as follows. A set of points is constructed across the
full d-dimensional parameter space, and implausibility is calculated at each point.
The points are collected into groups based on their placement in a projection to a
one- or two-dimensional slice of the parameter space. For each group, the proportion
of non-implausible points is calculated, and this value in [0,1] is plotted. The
minimum implausibility plots are similar, but with minimum implausibility calculated
rather than proportion of non-implausible points.
The maxpoints argument is used as a cutoff for if a regular ppd grid would
result in a very large number of points. If this is the case, then maxpoints points
are sampled uniformly from the region instead of regularly spacing them.
If only a subset of parameters are relevant, then the plot_vars and fixed_vars
can be used to specify the subset. If plot_vars is provided, corresponding to a list
of parameter names, then those parameters not included are fixed to their mid-range values;
if fixed_vars is provided as a named list, then the parameters not included are fixed
to the corresponding specified values.