title: Text not vertically centered in UILabel
body: <p>I've created a Label with the following code : </p>
  
  <pre><code>func setupValueLabel() {
      valueLabel.numberOfLines = 1
      valueLabel.font = UIFont(name: "Avenir-Black", size: 50)
      valueLabel.adjustsFontSizeToFitWidth = true
      valueLabel.clipsToBounds = true
      valueLabel.backgroundColor = UIColor.greenColor()
      valueLabel.textColor = valuesColor
      valueLabel.textAlignment = NSTextAlignment.Center
  }
  </code></pre>
  
  <p>I don't really understand why but the label is not vertically centered : 
  <img src="https://i.stack.imgur.com/k8B6c.png" alt="Label not centered"></p>
  
  <p>Do I have to do anything specific so it can be centered ?</p>