title: d3 - axis ticks not working unlike tickValues
body: <p>I made bar chart that has x-axis on time based and It worked with tickValues().</p>
  
  <pre><code>xAxis.tickFormat( customTimeFormat )
       .tickValues( xScale.domain()
       .filter( function( d, i ) {
            // return true or false by some process
       });
  </code></pre>
  
  <p>It works well as I expect, but when I made axis with ticks, it didn't work.</p>
  
  <pre><code>xAxis.tickFormat( customTimeFormat ).ticks(4);
  </code></pre>
  
  <p>How should I set ticks in axis?</p>