ax.axis('equal')
ax.quiver((0, 0), (0, 0), (3, 1), (1, 0), angles='xy', scale_units='xy',
    scale=1, linestyle='dashed', facecolor='none', linewidth=1)
ax.axis([-4, 4, -4, 4])
plt.show()
Plotting dashed 2D vectors with matplotlib?