<p>That one puzzled me several times, too, on another site.  It turns out there is an additional requirement over which you have little control: a tag has to appear in at least 100 threads in order to qualify for badges.  Presently, <a href="https://stats.stackexchange.com/questions/tagged/histogram" class="post-tag" title="show questions tagged &#39;histogram&#39;" rel="tag">histogram</a> has been used only 93 times.  Have patience.</p>
