title: sns.regplot shows a shaded area for the regression which does not make sense
body: <p>Given:</p>
  
  <pre><code>f = pd.DataFrame({"a": [1,2,3], "b": [1,2,3]})
  ax = sns.regplot(x="a", y="b", data=f)
  </code></pre>
  
  <p>I get this graph which is confusing as the points are perfectly lined up. Why would there be the shaded area? I guess I dont understand what the shading means exactly.</p>
  
  <p><a href="https://i.stack.imgur.com/0iOQc.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/0iOQc.png" alt="enter image description here"></a></p>