title: Plotnine group facet_grid labels
body: <p>Is there a way to group labels when using plotnine's <code>facet_grid</code> function?</p>
  <p>For example, take the following DataFrame (I made the example up such that you can reproduce my problem. In real my DataFrame is much bigger but the structure is almost identical):</p>
  <pre><code>df_test = pd.DataFrame(data= {'rate_1': [0.0,0.1,0.2,0.3,0.0,0.1,0.2,0.3,0.3,0.2,0.1,0.0],
                                'rate_2': [0.0,0.1,0.2,0.3,0.0,0.1,0.2,0.3,0.3,0.2,0.1,0.0],
                                'rate_3':[0.0,0.1,0.2,0.3,0.0,0.1,0.2,0.3,0.3,0.2,0.1,0.0],
                                'rate_4': [0.0,0.1,0.2,0.3,0.1,0.2,0.3,0.0,0.2,0.3,0.0,0.1],
                                'samples': [50000, 100000, 50000, 100000,50000, 100000, 50000, 100000,50000, 100000, 50000, 100000],
                                'model': ['model 1', 'model 2', 'model 3', 'model 4','model 1', 'model 2', 'model 3', 'model 4','model 1', 'model 2', 'model 3', 'model 4'],
                                'metric': [0.5,0.4,0.3,0.2,0.5,0.4,0.3,0.2,1,0.3,0.4,0.3]})
  </code></pre>
  <p>I created the following function to plot the absolute metric depending on the rates, sample size and model:</p>
  <pre><code># Plotting
  my_plot = ggplot(data=df_test, mapping=aes('rate_1', 'rate_2', fill='metric'))
  my_plot += geom_tile()
  my_plot += theme(axis_text_x=element_text(rotation=90), figure_size=(16, 8), strip_background_x=element_text(width=1.))
  my_plot += scale_x_continuous(breaks=[.0, .1, .2, .3, .4, .5])
  my_plot += scale_y_continuous(breaks=[.0, .1, .2, .3, .4, .5])
  my_plot += facet_grid('rate_3 + samples  ~ model + rate_4', labeller=label_value)
  my_plot += scale_fill_gradient2(midpoint=0, low='blue', mid=&quot;white&quot;, high=&quot;red&quot;)
  my_plot
  </code></pre>
  <p>The resulting plot looks like:</p>
  <p><a href="https://i.stack.imgur.com/o3zIL.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/o3zIL.png" alt="enter image description here" /></a></p>
  <p>Is there a way to group model names (model 1, model 2, model 3, model 4) and rate_4 when labeling the facet_grid? I'm looking for a result such that the 'columns' are named like the following (I used Excel for the illustration):</p>
  <p><a href="https://i.stack.imgur.com/rbQVp.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/rbQVp.png" alt="enter image description here" /></a>
  Thanks!</p>