ternal-link">mean</a></span><span class="op">(</span><span class="va">arr_delay</span><span class="op">)</span>, <span class="fu"><a href="https://rdrr.io/r/base/mean.html" class="external-link">mean</a></span><span class="op">(</span><span class="va">dep_delay</span><span class="op">)</span>,</span>
<span>               by <span class="op">=</span> <span class="fu">.</span><span class="op">(</span><span class="va">origin</span>, <span class="va">dest</span>, <span class="va">month</span><span class="op">)<