title: Plotting a Line Plot in Pandas on a double-indexed dataframe
body: <p>I have a dataframe as shown below with double index 'month' and 'day'</p>
  
  <pre><code>    query_sorted_gross = '''
      SELECT days.Gross, days.month, days.day
      FROM days, horror_movies
      WHERE horror_movies."Movie ID" = days."Movie ID"
  
  '''
  df_q1 = pd.read_sql_query(query_sorted_gross, conn)
  df_q1_grouped = df_q1.groupby(['month', 'day']).mean()
  </code></pre>
  
  <p>I want to create a line plot that shows date on the X-axis and Gross on the Y-axis. How do I plot a double-indexed dataframe with a line plot.</p>