title: Show groups with 0 group_by dplyr
body: <p>I have a dataframe eg.</p>
  
  <pre><code>a&lt;-data.frame(
     year=c(2001,2001,2002,2004,2001,2002,2004),
     binary=c(1,0,1,0,1,0,0))
  </code></pre>
  
  <p>I try for example:</p>
  
  <pre><code>a %&gt;% group_by(year) %&gt;% count()
  </code></pre>
  
  <p>I would like the output:</p>
  
  <pre><code>year    binary   n
  2001    0        1
  2001    1        1
  2002    1        1
  2002    0        1
  2004    0        1
  2004    1        0
  </code></pre>
  
  <p>I want to plot how many samples have the 1 phenotype but group_by deos not show a 1 group for 2004 because it has count 0.  How can I make it do so?</p>