title: Insert a dot at a certain point on a line with matplotlib
body: <p>I wish to know how to insert a dot (or some kind of marker) at a distinct point on a curve/line in matplotlib. Using the tutorial documentation, <a href="http://matplotlib.org/users/pyplot_tutorial.html" rel="noreferrer">http://matplotlib.org/users/pyplot_tutorial.html</a></p>
  
  <p>we plot</p>
  
  <pre><code>import matplotlib.pyplot as plt
  plt.plot([1,2,3,4], [1,4,9,16])
  plt.axis([0, 6, 0, 20])
  plt.show()
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/j50AW.png" alt="enter image description here"></p>
  
  <p>Now, I know how to transform this line into a series of points, here using red dots <code>'ro'</code>:</p>
  
  <pre><code>import matplotlib.pyplot as plt
  plt.plot([1,2,3,4], [1,4,9,16], 'ro')
  plt.axis([0, 6, 0, 20])
  plt.show()
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/Tq9OE.png" alt="enter image description here"></p>
  
  <p>How can I add a "dot" at a distinct point? For example, add a dot at point <code>[3,9]</code>?</p>