title: Custom tick size on axis (d3js)
body: <p>I am creating a svg x-y-chart in d3.js. Is it possible to create ticks of different lengths depending on tickValue?</p>
  
  <p>I have made my own tickFormat function <code>myTickFormat</code> and use that in <code>.tickFormat([format])</code> and that works fine because <em>[format]</em> is expected to be a function. But it is not possible to do the same with <code>.innerTickSize([size])</code>, which expects a number. </p>
  
  <p>E.g. if I want the tick at value 70 to be longer I want to do something like this:</p>
  
  <pre><code>var myTickSize = function(d) {
      if (d === 70) { return 20;}
      return 6;
  };
  </code></pre>
  
  <p>But when I use <code>myTickSize</code> as argument to <code>.innerTickSize()</code>:</p>
  
  <pre><code>var yScale = d3.scale.linear();
  var yAxis = d3.svg.axis()
                 .scale(yScale).orient("left")
                 .innerTickSize(myTickSize);
  </code></pre>
  
  <p>I get an <strong>Error: Invalid value for  attribute x2="NaN"</strong> error for each tick.</p>