'Creates x and y startpoint and endpoint pairs
After finding the endpoint of each barb this zips startpoint and
endpoint pairs to create 2 lists: x_values for barbs and y values
for barbs
:rtype: (list, list) barb_x, barb_y: list of startpoint and endpoint
x_value pairs separated by a None to create the barb of the arrow,
and list of startpoint and endpoint y_value pairs separated by a
None to create the barb of the arrow.'

def get_barbs(self):

    self.end_x = [(i + j) for (i, j) in zip(self.x, self.u)]
    self.end_y = [(i + j) for (i, j) in zip(self.y, self.v)]
    empty = ([None] * len(self.x))
    barb_x = utils.flatten(zip(self.x, self.end_x, empty))
    barb_y = utils.flatten(zip(self.y, self.end_y, empty))
    return (barb_x, barb_y)
