title: How to mark specific data points in matplotlib graph
body: <p>I have a plot that I want to mark some values of x on the graph like in the following image, (p.s. I put the dots by hand)</p>
  <p><img src="https://i.stack.imgur.com/92yub.png" alt="see the plot" /></p>
  <p>I tried the following code, yet it did not work as I have expected.</p>
  <pre><code>roots = [-1,1,2]
  plt.plot(vals,poly,markevery=roots,label='some graph')
  </code></pre>
  <p>As a wrap up, I want to put a dot on the function line which indicates that point is the root.</p>