title: How to put labels in a stacked bar using plotnine
body: <p>I have the following <code>dataframe</code></p>
  
  <pre><code>import pandas as pd
  from plotnine import *
  
  df = pd.DataFrame({
      'variable': ['gender', 'gender', 'age', 'age', 'age', 'income', 'income', 'income', 'income'],
      'category': ['Female', 'Male', '1-24', '25-54', '55+', 'Lo', 'Lo-Med', 'Med', 'High'],
      'value': [60, 40, 50, 30, 20, 10, 25, 25, 40],
  })
  df['variable'] = pd.Categorical(df['variable'], categories=['gender', 'age', 'income'])
  </code></pre>
  
  <p>An I am using the following code to get the stacked bar plot</p>
  
  <pre><code>(ggplot(df, aes(x='variable', y='value', fill='category'))
   + geom_col()
  )
  </code></pre>
  
  <p>The above code was taken from <a href="https://plotnine.readthedocs.io/en/stable/generated/plotnine.geoms.geom_col.html" rel="nofollow noreferrer">here</a></p>
  
  <p>How can I put labels, ( in this case the <code>value</code>s ), in the middle of each category at each bar ?</p>