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

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.

...:

graphical parameters can be specified to control drawing of the fitted line. By default, parameters lwd, lty, and col are gotten from the trellis parameter list add.line.