Add Reference Grid to Panels

USAGE:

panel.grid(h = 3, v = 3) 

ARGUMENTS:

h
the number of horizontal reference lines which evenly divide the panel. If h is 0, no horizontal reference lines will be drawn. If h is negative, there will be horizontal lines that align with the horizontal tick marks.
v
the number of vertical reference lines which evenly divide the panel. If v is 0, no vertical reference lines will be drawn. If v is negative, there will be vertical lines that align with the vertical tick marks.
...
graphical parameters may be supplied to control the reference lines. By default, values of parameters lwd, lty , and col are used from object reference.line.

SIDE EFFECTS:

The function adds horizontal and/or vertical reference lines to a plot.

EXAMPLES:

panel.grid(h = -1, v = -1)  # grid aligned with tick marks