Preliminary Computations to Add Smooth Loess Curve

USAGE:

prepanel.loess(x, y, span = 2/3, degree = 1, family = "symmetric", 
               evaluation = 50, ...) 

ARGUMENTS:

x
horizontal coordinates of data points.
y
vertical coordinates of data points.
span
smoothing parameter.
degree
overall degree of locally-fitted polynomial. 1 is locally-linear fitting and 2 is locally-quadratic fitting.
family
allowable values are "gaussian" or "symmetric". In the first case, local-fitting methods are used. In the second case, the default, local fitting is used together with a robustness feature that guards against distortion by outliers.
evaluation
number of values at which the loess curve is evaluated.

VALUE:

A list with components
dx
the change in x of each of the line segments making up the fit. Together with dy, these segments are used in banking computations to determine the aspect ratio of the Trellis plot.
dy
the change in y of each of the line segments making up the fit.
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 smooth curve.
ylim
similar to xlim, but dealing with the y coordinates of the original data and the fit.