title: Calculate Percentage for each time series observations per Group in R
body: <p>New to R,  so just my getting head around the data wrangling aspect. Tried looking for a similar question but couldn't find it. </p>
  
  <p>I would like to add an additional column that is the percentage of views split for each day between the article groups. Example Dataset below</p>
  
  <pre><code>  views       date      article
    1578   2015-01-01       A
    616    2015-01-01       B
    575    2015-01-01       C
    1744   2015-01-02       A
    541    2015-01-02       B
    660    2015-01-02       C
    2906   2015-01-03       A
    629    2015-01-03       B
    643    2015-01-03       C    
  </code></pre>
  
  <p>And the expected result I am looking for.. </p>
  
  <pre><code> views     percentage   date           article
    1578     56.99        2015-01-01       A
    616      22.25        2015-01-01       B
    575      20.77        2015-01-01       C
    1744     59.22        2015-01-02       A
    541      18.37        2015-01-02       B
    660      22.41        2015-01-02       C
    2906     69.55        2015-01-03       A
    629      15.06        2015-01-03       B
    643      15.39        2015-01-03       C  
  </code></pre>
  
  <p>I know this is possible by splitting the date frame using subsets but I would hope there is more neat approach using a library ? </p>
  
  <p>Thanks ! </p>