title: get proportion of each group's value counts after group_by dplyr R
body: <p>In python pandas I was able to simply do <code>df.groupby(x,y).value_counts(normalize=True)</code> to get the proportion of each value in a group. However I've been unable to find a way to do this in R.</p>
  
  <p>I've grouped my df by x and y and have summarized to calculate the frequency,
  as such <code>df %&gt;% group_by(x,y) %&gt;% summarize(count=n())</code> but I'd like to instead see the proportion of each y for each x.</p>
  
  <pre><code>x   y  count
  1   A   22      
  1   B   65      
  1   C   94      
  1   D   40      
  2   D   34      
  2   E   1       
  2   F   6       
  3   E   4       
  3   F   13      
  </code></pre>
  
  <p>for example, the new column of proportions should have</p>
  
  <pre><code>x   y   proportion
  1   A   0.0995475
  1   B   0.2941176
  1   C   0.4253393
  1   D   0.1809955
  2   D   0.8292683
  2   E   0.024390
  2   F   0.1463415
  3   E   0.2352941
  3   F   0.7647059
  </code></pre>