np.random.seed(12345)
dates = pd.date_range(start='1/1/2008', periods=10, freq='Y')
df = pd.DataFrame({'x':np.arange(10), 'y':np.random.random(size=(10,)), 'dates':dates})

fig, ((ax1, ax2), (ax3, ax4), (ax5, ax6)) = plt.subplots(3,2, sharex='col', sharey=True, figsize=(10,10))

# no datetime, xycoords='data'
ax1.set_title('no datetime', fontweight='bold')
ax1.set_ylabel('xycoords=data', fontweight='bold')
ax1.plot(df['x'], df['y'])
ax1.annotate("test1", xy=(df.loc[5,'x'], 0.5), xycoords='data',
                     )
ax1.annotate("test2", xy=(df.loc[6,'x'], 0.5), xycoords='data',
                     xytext=(50,50), textcoords='offset pixels', arrowprops=dict(arrowstyle='->'))

# no datetime, xycoords='axes fraction'
ax3.set_ylabel('xycoords=fraction', fontweight='bold')
ax3.plot(df['x'], df['y'])
ax3.annotate("test1", xy=(0.5, 0.5), xycoords='axes fraction',
                     )
ax3.annotate("test2", xy=(0.6, 0.6), xycoords='axes fraction',
                     xytext=(50,50), textcoords='offset pixels', arrowprops=dict(arrowstyle='->'))

# no datetime, xycoords=('data', 'axes fraction')
ax5.set_ylabel('xycoords=mixed', fontweight='bold')
ax5.plot(df['x'], df['y'])
ax5.annotate("test1", xy=(df.loc[5,'x'], 0.5), xycoords=('data','axes fraction'),
                     )
ax5.annotate("test2", xy=(df.loc[6,'x'], 0.5), xycoords=('data','axes fraction'),
                     xytext=(50,50), textcoords='offset pixels', arrowprops=dict(arrowstyle='->'))


# datetime, xycoords='data'
ax2.set_title('datetime', fontweight='bold')
ax2.plot(df['dates'], df['y'])
ax2.annotate("test1", xy=(df.loc[5,'dates'], 0.5), xycoords='data',
                     )
ax2.annotate("test2", xy=(df.loc[6,'dates'], 0.5), xycoords='data',
                     xytext=(50,50), textcoords='offset pixels', arrowprops=dict(arrowstyle='->'))

# datetime, xycoords='axes fraction'
ax4.plot(df['dates'], df['y'])
ax4.annotate("test1", xy=(0.5, 0.5), xycoords='axes fraction',
                     )
ax4.annotate("test2", xy=(0.6, 0.6), xycoords='axes fraction',
                     xytext=(50,50), textcoords='offset pixels', arrowprops=dict(arrowstyle='->'))

# datetime, xycoords='axes fraction'
ax6.plot(df['dates'], df['y'])
ax6.annotate("test2", xy=(df.loc[6,'dates'], 0.5), xycoords=('data','axes fraction'),
                     xytext=(50,50), textcoords='offset pixels', arrowprops=dict(arrowstyle='->'))
ax6.annotate("test1", xy=(df.loc[5,'dates'], 0.5), xycoords=('data','axes fraction'),
                     )

fig.autofmt_xdate()

ax6.annotate("test2", xy=(df.loc[6,'dates'], 0.5), xycoords=('data','axes fraction'),
                     xytext=(50,50), textcoords='offset pixels', arrowprops=dict(arrowstyle='->'))
ax6.annotate("test1", xy=(df.loc[5,'dates'], 0.5), xycoords=('data','axes fraction'),
                     )