title: Set multiple titles for a matplotlib subplot
body: <p>I'm creating subplots of venn diagrams that are made via <a href="https://github.com/konstantint/matplotlib-venn" rel="nofollow noreferrer"><code>matplotlib_venn</code></a>. Here is the plot I'm making:</p>
  
  <p><img src="https://i.stack.imgur.com/T4cNy.png" alt="Subplots missing top-right title"></p>
  
  <p>Notice the missing title on the top-right plot. This is because whenever I call <code>ax.set_title</code>, it's replacing existing subplot titles. The code to create this plot was:</p>
  
  <pre><code>oadoi_colors = ['all', 'closed', 'closed + green', 'bronze', 'green', 'hybrid', 'gold']
  collections = ['Combined', 'Crossref', 'Unpaywall', 'Web of Science']
  figure, axes = matplotlib.pyplot.subplots(nrows=len(oadoi_colors), ncols=len(collections), figsize=(2.7 * len(collections), 2.7 * len(oadoi_colors)))
  subplot_label_rectprops = {'facecolor': '#fef2e2', 'boxstyle': 'round'}
  
  for (oadoi_color, collection), df in coverage_df.groupby(['oadoi_color', 'collection']):
      i = oadoi_colors.index(oadoi_color)
      j = collections.index(collection)
      ax = axes[i, j]
      venn_plotter(df, ax=ax)
      if i == 0:
          text = ax.set_title(collection, loc='center', y=1.1)
          text.set_bbox(subplot_label_rectprops)
          # Top-right subplot cannot titled twice: https://stackoverflow.com/questions/36602347
      if j == len(collections) - 1:
          text = ax.set_title(oadoi_color.title(), x=1.1, y=0.5, verticalalignment='center', rotation=270)
          text.set_bbox(subplot_label_rectprops)
  </code></pre>
  
  <p><code>venn_plotter</code> is a function that calls <code>matplotlib_venn.venn3_unweighted</code>. <code>df</code> is a <code>pandas.DataFrame</code> of the data.</p>
  
  <p>In the matplotlib <a href="https://matplotlib.org/examples/pylab_examples/titles_demo.html" rel="nofollow noreferrer">title demo</a>, it seems like you can set multiple titles... but I can't figure out how to do this with subplots.</p>
  
  <p>There's an <a href="https://stackoverflow.com/a/36602846/4651668">answer to a similar question</a> that suggests using <code>set_ylabel</code>. However, setting the ylabel has no effect on these venn plots.</p>