title: Set ticks according to formula
body: <p>Given a figure created by spectrogram function, how do I set the ticks according to the following formula?</p>
  
  <pre><code>(12 × log2 (f / 440)) + 69
  </code></pre>
  
  <p>[<code>f</code> is the current value on the axis, and the formula's result should replace it]</p>