import pyqplot as qp
import numpy as np

qp.figure('line', 3, 3)

xx = np.linspace(-np.pi, np.pi, 50)
yy = np.sin(xx)

qp.plot(xx, yy)

for n in range(2, len(xx)-1, 5):
    qp.at(xx[n], yy[n], xx[n+1]-xx[n], yy[n+1]-yy[n])
    qp.line([0, 0],[-5, 5])