  pljoin(x1, y1, x2, y2)
  **********************

  pljoin

  Joins the point (x1, y1) to (x2, y2).

  x1 (PLFLT, input): x coordinate of first point.

  y1 (PLFLT, input): y coordinate of first point.

  x2 (PLFLT, input): x coordinate of second point.

  y2 (PLFLT, input): y coordinate of second point.

