x = [dt.datetime(2009, 5, 1), dt.datetime(2010, 6, 1), dt.datetime(2011, 4,
    1), dt.datetime(2012, 6, 1)]
y = [1, 3, 2, 5]
fig, ax = plt.subplots()
ax.plot_date(x, y, linestyle='--')
ax.annotate('Test', (mdates.date2num(x[1]), y[1]), xytext=(15, 15),
    textcoords='offset points', arrowprops=dict(arrowstyle='-|>'))
fig.autofmt_xdate()
plt.show()
Annotate Time Series plot in Matplotlib