title: Create summary table with averages of column subsets
body: <p>I am very new to this so be please nice!</p>
  <p>My dataset has 3 columns - Col A (chr, 'X' and 'Y'), Col B (chr, 'red', 'blue' 'yellow'), Col C (numeric).</p>
  <pre><code>Col_A &lt;- c('X', 'X', 'X', 'Y', 'Y', 'X', 'Y', 'X')
  Col_B &lt;- c('red', 'yellow', 'blue', 'blue', 'yellow', 'red', 'blue', 'red')
  Col_C &lt;- c(12, 22, 13, 14, 25, 17, 11, 10)
  
  dataset &lt;- as.data.frame(cbind(Col_A, Col_B, Col_C))
  </code></pre>
  <p>How can I create a summary table in R that's like</p>
  <pre><code>tbl_summary(dataset, by = 'Col_A') 
  </code></pre>
  <p><img src="https://i.stack.imgur.com/sokis.png" alt="output shown here" /></p>
  <p>but gives the mean values of Col C for every subgroup in Col B?
  <img src="https://i.stack.imgur.com/hToys.png" alt="enter image description here" /></p>
  <p>Right now the code gives me counts for Col B and then the mean value of Col C for 'X' and 'Y'.</p>
  <p>Does any of that even make sense?!</p>