title: Using seaborn, how can I draw a line of my choice across my scatterplot?
body: <p>I want to be able to draw a line of my specification across a plot generated in seaborn.  The plot I chose was JointGrid, but any scatterplot will do.  I suspect that seaborn maybe doesn't make it easy to do this?</p>
  
  <p>Here is the code plotting the data (dataframes from the Iris dataset of petal length and petal width):</p>
  
  <pre><code>import seaborn as sns
  iris = sns.load_dataset("iris")    
  grid = sns.JointGrid(iris.petal_length, iris.petal_width, space=0, size=6, ratio=50)
      grid.plot_joint(plt.scatter, color="g")
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/9II9Z.png" rel="noreferrer"><img src="https://i.stack.imgur.com/9II9Z.png" alt="enter image description here"></a></p>
  
  <p>If you take this graph from the iris dataset, how can I draw a line of my choice across it?  For example, a line of negative slope might separate the clusters, and positive slope might run across them.</p>