title: How to properly use FuncFormatter(func)?
body: <p><a href="http://matplotlib.org/api/ticker_api.html#matplotlib.ticker.FuncFormatter" rel="noreferrer">class matplotlib.ticker.FuncFormatter(func)</a>
  The function should take in two inputs (tick value x and position pos) and return a string</p>
  
  <pre><code>def millions(x, pos):
      'The two args are the value and tick position'
      return '$%1.1fM' % (x*1e-6)
  </code></pre>
  
  <p>What happened to the pos parameter? It isn't even set to None. </p>
  
  <p>I added print(pos) and got 0 1 2 3 4, plus a lot of None when I moved my mouse over the image. Only I don't know what to do with that information. </p>
  
  <p>I have seen examples where <a href="https://stackoverflow.com/questions/25973581/how-to-format-axis-number-format-to-thousands-with-a-comma-in-matplotlib">x is used</a> but not pos, and I don't understand how it is supposed to be used. Can someone give me an example? Thanks</p>