title: Show every other tick label on d3 time axis?
body: <p>I've got a d3 chart with an x-axis that uses a custom time format:</p>
  
  <pre><code>var x = d3.time.scale.utc()
          .domain([start, end])
          .range([0, width]);
  
  var customTimeFormat = d3.time.format.utc.multi([
      ["%b %d", function(d) { return d.getUTCDate() != 1; }],
      ["%b", function(d) { return d.getUTCMonth(); }],
      ["%Y", function() { return true; }]
  ]);
  
  var xAxisTop = d3.svg.axis()
                  .scale(x)
                  .orient("top")
                  .tickFormat(customTimeFormat)
                  .innerTickSize(-height)
                  .outerTickSize(0)
                  .ticks(d3.time.month.utc, 1);
  </code></pre>
  
  <p>What I'd like to be able to do is to have a tick for each month, but only a label for each third month. All I seem to be able to do, however, is to either (a) have a tick and label for each month or (b) have a tick and label for every third month.</p>
  
  <p>How can I specify to draw ticks every month, but only have the labels show up for every third month?</p>