This function plots the smooth coefficients of a pfr object. These include
functional coefficients as well as any smooths of scalar covariates. The
function dispatches to `pfr_plot.gam`

, which is our local copy of
`plot.gam`

with some minor changes.

```
# S3 method for pfr
plot(x, Qtransform = FALSE, ...)
```

x

a fitted `pfr`

-object

Qtransform

For additive functional terms, `TRUE`

indicates the
coefficient should be plotted on the quantile-transformed scale, whereas
`FALSE`

indicates the scale of the original data. Note this is
different from the `Qtransform`

arguemnt of `af`

, which specifies
the scale on which the term is fit.

...

arguments handed over to `plot.gam`

This function's main purpose is its side effect of generating plots. It also silently returns a list of the data used to produce the plots, which can be used to generate customized plots.