title: Modify seaborn line relplot legend title
body: <p>First off, I am a newbie with both Python and seaborn, but after spending some time with this matter I believe my question has not been addressed elsewhere. If so, I sincerely apologize.</p>
  
  <p>I want to use the seaborn <code>relplot()</code> function with <code>kind='line'</code> to plot data from a dataframe as several lines of different colors. The data corresponding to the lines are separated categorically by values in a column (<code>cat</code>, say) and I use <code>hue</code> (in this case, <code>hue='cat'</code>). Here is a simple example:</p>
  
  <pre><code>import pandas as pd
  import seaborn as sns
  df = pd.DataFrame({'x': [0, 1, 3, 4, 7, 1, 2, 5, 6],
                     'y': [2, 4, 3, 6, 5, 7, 10, 9, 7],
                     'cat': [0, 0, 0, 0, 0, 1, 1, 1, 1]})
  g = sns.relplot(x='x', y='y', hue='cat', kind='line', data=df);
  </code></pre>
  
  <p>It produces the following graph:</p>
  
  <p><img src="https://i.stack.imgur.com/bh03B.png" alt="Example plot"></p>
  
  <p>How do I modify the legend title without changing the name of the column <code>cat</code>? I believe my problem is that the legend title is actually implemented as a legend item, so what I want to do is remove that item and then add a proper legend title (which I can do using <code>g._legend.set_title('New title')</code>), but I do not know how to accomplish this.</p>
  
  <p>This question differs from <a href="https://stackoverflow.com/q/51579215/10595482">Remove seaborn lineplot legend title</a> in that <code>relplot()</code> with <code>kind='line'</code> produces a <code>FacetGrid</code> figure and attaches the legend to that one. This means I cannot access the legend content as <code>g.ax.legend().texts</code> but must use <code>g._legend</code>, in which case I am lost.</p>