title: Center two fonts with different different sizes vertically in an NSAttributedString
body: <p>I use <code>NSAttributedString</code> to generate a string with two different sizes. By default, its bottom alignment looks like this:</p>
  
  <p><img src="https://i.stack.imgur.com/CmMcd.png" alt="baseline aligned sizes"></p>
  
  <p>But I want to center it vertically, like this:
  <img src="https://i.stack.imgur.com/jb9D6.png" alt="vertically centered sizes"></p>
  
  <p>To be clear, this is a single attributed string, not two or more. This is a simplified example to describe my question, what I'd actually like to do is more complex.</p>