title: D3: tickformat method to only show year
body: <p>My dates for my d3 chart are month/day/year, for example: "10/1/2013"</p>
  
  <p>And I'm parsing them this way: d3.time.format("%x").parse;</p>
  
  <p>But how do I write a .tickFormat() method on my axis to only show the year (full year, with century)?</p>