title: D3 - access axis tick by index
body: <p><br/>
  I have an histogram drawn in D3. X-scale is ordinal.<br/>
  When I hover specific datum on histogram with mouse cursor, I want the font-weight of corresponding tick text label on X-axis to become bold.<br/>
  For that purpose I use</p>
  
  <pre><code>.on("mouseover", highlightTickCallback)
  </code></pre>
  
  <p>callback.<br/> 
  The highlightTickCallback function receives corresponding datum as first parameter, so I could calculate tick index by searching specific key in domain of X-scale. But now, when I have index of tick, I need to access this tick somehow, and I don't understand how could I do that.</p>