This is a subclass of GD::Polygon used to draw an arrow between two vertices.

GD::Arrow::Full draws a full arrow between two verticies.

                                  |\
           +----------------------+ \
  (X2, Y2) *                         * (X1, Y1)
           +----------------------+ /
                                  |/

GD::Arrow::RightHalf draws a half arrow between two verticies.

  (X2, Y2) *-------------------------* (X1, Y1)
           +----------------------+ /
                                  |/

GD::Arrow::LeftHalf draws a half arrow between two verticies.

                                  |\
           +----------------------+ \
  (X2, Y2) *-------------------------* (X1, Y1)