a quantile function for the reference distribution, e.g. qnorm.
This argument is automatically supplied by qqmath.
VALUE:
A list with components
dx:
the change in x of the fitted line.
This line is used in banking computations
to determine the aspect ratio of the Trellis plot.
dy:
the change in y of the fitted line.
xlim:
a vector of length 2 giving the minimum and maximum values for the
x coordinates on the plot, taking into account the original data
as well as the fitted line.
ylim:
similar to xlim, but dealing with the y coordinates of the original
data and the fit.