ax.plot(x, y, 'bo')
ax.annotate('Something', xy=(x[0], y[0]), xytext=(-20, 20), textcoords=
    'offset points', ha='center', va='bottom', bbox=dict(boxstyle=
    'round,pad=0.2', fc='yellow', alpha=0.3), arrowprops=dict(arrowstyle=
    '->', connectionstyle='arc3,rad=0.5', color='red'))
plt.show()
matplotlib: how to annotate point on a scatter automatically placed arrow?