After eliminating out of range p--values
and missing values produces a Q--Q plot.
If the package qqconf is unavailable or
plotversion is set other than "qqconf"
then the plot uses the function from base graphics.
The line shown is
through the quantiles (by default the 0.25 and 0.75).
If the package qqconf is available then the plot
uses that package's function and as well as the Q--Q plot
produces a confidence region.
Parameters can be passed to qq_conf_plot to
exert further control over the plot.
If plotversion is set to "qqconf"
but package qqconf is not available
then a warning is given and no plot is produced.