title: Plotnine custom bar polt colors
body: <p>I am trying to plot two overlay bar plots over 4 different categories. My dataset looks something like:</p>
  
  <pre><code>category count_first count_second
      a          15         80
      b          18         50
      c          12         99
      d          11         30
  </code></pre>
  
  <p>I am trying to plot a bar plot for both count_first, as well as count second whilst perserving the bar order by count. If not ordered, I can simply plot this with dataframe.melt('category')... but I am having problems with the case, where the bars need to be ordered.</p>
  
  <pre><code>df = df.sort_values(by=['category'])
  m_categories=reversed(list(df['category']))
  df['category'] = pd.Categorical(df['category'],categories=m_categories, ordered=False)
  print(df)
  
  #df2 = df.melt('category') -- DOESNT work
  
  gx = (ggplot(df)
        + geom_col(aes('category','count_first', fill="#FFCE33"),stat="identity") + theme_classic()
        + geom_col(aes('category','count_second'),stat="identity")
        + xlab("category")
        + ylab("counts")
        + theme(axis_text=element_text(size=12,angle=70),
                axis_title=element_text(size=15))
  )
  gx.draw()
  plt.show()
  </code></pre>
  
  <p>I cant seem to set the fill parameter for individual geom_col objects, how can this be done?</p>