title: Plot DataFrame groupby with multiindex for a specific index
body: <p>I am trying to plot some data coming from a series of executions. I massaged the data to get a structure which "feels good".</p>
  
  <p>The table is multiindex. There is a <code>nodes</code> index, which is the one I want in the plot, and a <code>size</code> index, which will be <code>groupby</code>-ed. I arrived to this structure by the following:</p>
  
  <pre><code>ws_data = df.query("...").pivot_table(index=["nodes", "size"], ...).groupby(level="size")
  
  ws_data.plot(y="time")
  </code></pre>
  
  <p>However, the x axis has the tuple <code>(nodes, size)</code>, which I would like to avoid. I only need to paint the nodes, which is the variable that changes in a single plot, and I want to use the <code>size</code> for the plot title.</p>
  
  <p>I have been struggling with flags of the <a href="https://pandas.pydata.org/pandas-docs/version/0.16.2/generated/pandas.core.groupby.DataFrameGroupBy.plot.html" rel="nofollow noreferrer">groupby.Plot</a> as well as if <a href="https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.groupby.html" rel="nofollow noreferrer">groupby</a> can simplify or drop that intermediate index.</p>
  
  <p>Am I doing something very wrong? Or I am overlooking something? It feels like I am near my result, but maybe I am misusing the tools. </p>
  
  <hr>
  
  <p>To simulate the structure and the plot I am having:</p>
  
  <pre><code>df = pd.DataFrame(
      {"time": np.random.random(6)},
      index=pd.MultiIndex.from_product(
          [[1, 2, 4], [4096, 8192]], 
          names=['nodes', 'size']
      )
  )
  
  ws_data = df.groupby(level="size")
  ws_data.plot(y="time")
  </code></pre>