Given a single emulator, or a set of emulators, the emulator statistics can be plotted
across a two-dimensional slice of the parameter space. Which statistic is plotted is
determined by plot_type: options are `exp', `var', `sd', `imp', and `nimp', which
correspond to expectation, variance, standard deviation, implausibility, and nth-max
implausibility.
By default, the slice varies in the first two parameters of the emulators, and all other
parameters are taken to be fixed at their mid-range values. This behaviour can be changed
with the params and fixed_vals parameters (see examples).
If the statistic is `exp', `var' or `sd', then the minimal set of parameters to pass to this
function are ems (which can be a list of emulators or a single one) and plot_type.
If the statistic is `imp' or `nimp', then the targets must be supplied - it is not
necessary to specify the individual target for a single emulator plot. If the statistic is
`nimp', then the level of maximum implausibility can be chosen with the parameter nth.
Implausibility plots are typically coloured from green (low implausibility) to red (high
implausibility): a colourblind-friendly option is available and can be turned on by setting
cb = TRUE.
The granularity of the plot is controlled by the ppd parameter, determining the number
of points per dimension in the grid. For higher detail, at the expense of longer computing
time, increase this value. The default is 30.