polygon_df object from the given coordinatescode using polygon_df should not assume that the first and last point
within each id are the same. i.e. they may have to manulaly set a final
point equal to the initial point if that is what their graphics system
desires
create_polygon_df(x, y, id = 1L)coordinates of polygon. not necessarily closed.
a numeric vector used to separate locations in x,y into multiple polygons
data.frame with x, y, id columns.