title: time series plot for missing data
body: <p>I have some sequence event data for which I want to plot the trend of missingness on <code>value</code> across time. Example below:</p>
  <pre><code>        id    time  value
  1    aa122      1     1
  2   aa2142      1     1
  3   aa4341      1     1
  4    bb132      1     2
  5   bb2181      2     1
  6   bb3242      2     3
  7   bb3321      2     NA
  8    cc122      2     1
  9   cc2151      2     2
  10  cc3241      3     1
  11   dd161      3     3
  12  dd2152      3     NA
  13  dd3282      3     NA
  14   ee162      3     1
  15  ee2201      4     2
  16  ee3331      4     NA
  17  ff1102      4     NA
  18  ff2141      4     NA
  19  ff3232      5     1
  20   gg142      5     3
  21  gg2192      5     NA
  22  gg3311      5     NA
  23  gg4362      5     NA
  24   ii111      5     NA
  </code></pre>
  <p>The <code>NA</code> suppose to increase over time (the behaviors are fading). How do I plot the <code>NA</code> across time</p>