title: How to add annotation in a plot
body: <p>Given the following dataframe and I would like to add an error between the <code>expected</code> and the <code>actual</code> that shows the <code>rate_off</code></p>
  <pre><code>  Month  Expected  Actual  rate_off
  0   Jan        30      40     25.00
  1   Feb        25      23     -8.70
  2   Mar        50      51      1.96
  3   Apr        20      17    -17.65
  </code></pre>
  <p>so in my following code I get this graph:</p>
  <pre><code>import matplotlib.pyplot as plt
  import numpy as np 
  import pandas as pd
  
  df = pd.DataFrame({&quot;Month&quot;:['Jan','Feb','Mar','Apr'], 'Expected':[30, 25, 50, 20],'Actual':[40, 23, 51, 17]})     
  #df['rate_off'] = np.round(100-np.abs(df['Expected']/df['Actual']*100),2) 
   
  fig, ax = plt.subplots(1,1)
  df.plot(kind='bar', ax=ax)
  ax.legend()
  plt.show()
  </code></pre>
  <p><a href="https://i.stack.imgur.com/uta7g.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/uta7g.png" alt="enter image description here" /></a></p>
  <p>What I wish to add is some annotation that describe the difference (rate_off) to help the end user to get the data instantly.</p>
  <p>So a desired result would be:</p>
  <p><a href="https://i.stack.imgur.com/0Q9iH.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/0Q9iH.png" alt="enter image description here" /></a></p>