title: R plot frequency of strings with specific pattern
body: <p>Given a data frame with a column that contains strings. I would like to plot the frequency of strings that bear a certain pattern. For example</p>
  
  <pre><code>strings  &lt;- c("abcd","defd","hfjfjcd","kgjgcdjrye","yryriiir","twtettecd")
  df &lt;- as.data.frame(strings)
  df
       strings
  1       abcd
  2       defd
  3    hfjfjcd
  4 kgjgcdjrye
  5   yryriiir
  6  twtettec
  </code></pre>
  
  <p>I would like to plot the frequency of the strings that contain the pattern `"cd"
  Anyone with a quick solution?</p>