title: Add cursor to matplotlib
body: <p>This picture shows what I am trying to achieve:</p>
  
  <p><img src="https://i.stack.imgur.com/UceCf.png#" alt=""></p>
  
  <p>I am searching for a solution to add a cursor to my plotted line in <code>matplotlib</code>. The cursor should be draggable, but should only move on the plotted line. A label shall display the actual value of the marked point on the trace.</p>
  
  <p>I don't have an idea which object to use as this cursor/marker.</p>