2PLOT: plot rectangle - draw a rectangle

Usage: plot rectangle <x1> <y1> <x2> <y2>

draws a rectangle with corners (x1,y1), (x2,y1), (x2,y2) and (x1,y2).
This command can also be used in list mode, where x1, y1, x2 and y2
are four vector expressions. In list mode, the command can also be
used without parameters. In this case a histogram bar, is drawn for
each point in the list columns.
