<p><em>(Turning my comments into an official answer.)</em>  </p>

<p>I changed the name of the tag to <a href="https://stats.stackexchange.com/questions/tagged/acf-pacf" class="post-tag" title="show questions tagged &#39;acf-pacf&#39;" rel="tag">acf-pacf</a>.  </p>

<p>I am open to making it a synonym, but I'm also not convinced it should be a synonym of <code>[autocorrelation]</code> per se.  I don't know enough about the topic to be confident of making any definitive statements, but it seems to me that someone might want to ask a question specifically about the plots themselves and that allowing the organization of such material by having the tag can be useful.  In particular, they aren't really <em>synonyms</em>, in the literal sense.  That doesn't mean that they can't be made into tag synonyms, but does suggest we should think it through first.  For example, I could imagine someone asking, 'how to identify the MA parameter from this ACF plot?'.  I wouldn't want such a question to be silently retagged to <a href="https://stats.stackexchange.com/questions/tagged/autocorrelation" class="post-tag" title="show questions tagged &#39;autocorrelation&#39;" rel="tag">autocorrelation</a>.  </p>

<p>Moreover, as @amoeba notes, the tag has been used by others even in the short time since I created it.  That implies there may be some demand for it.  </p>

<p>Thus, while I remain open to making it a synonym of something, I lean towards leaving it alone for now.  </p>
