If there are no tuning parameters, or none were varied, an error is
produced.
If the model has one tuning parameter with multiple candidate values, a plot
is produced showing the profile of the results over the parameter. Also, a
plot can be produced if there are multiple tuning parameters but only one is
varied.
If there are two tuning parameters with different values, a plot can be
produced where a different line is shown for each value of of the other
parameter. For three parameters, the same line plot is created within
conditioning panels/facets of the other parameter.
Also, with two tuning parameters (with different values), a levelplot (i.e.
un-clustered heatmap) can be created. For more than two parameters, this
plot is created inside conditioning panels/facets.