Until now, most of the plots we have seen show only one data set.
Often, as in figure 3.1, on page , it may be
useful to plot two or more data sets together.
There are several ways to arrange more than one plot on a page or
window. This chapter shows first how you can overlay plots; how you
can use different plot styles and create legends (or keys) so that
overlaid plots can be distinguished; and finally how you can arrange
plots side-by-side on a page or window.