title: highcharts datetime x-axis custom formatting
body: <p>I would like to have just the first letter of each weekday as the x-axis, ie M T W T F S S repeated.  Currently you can set <code>dateTimeLabelFormats</code>, which I've set to use <code>%a</code>, which is the dateFormat for short weekday (Mon Tues Wed etc).  How can I just use the first letter?</p>
  
  <p>Here's my code (I'm using Lazy Highcharts in rails)</p>
  
  <pre><code> f.xAxis({type: 'datetime', tickInterval: 24*3600*1000, dateTimeLabelFormats: {
          day: '%a',
          week: '%a'}
          })
  </code></pre>
  
  <p>Thanks.</p>