title: how to add multiple vertical lines in a lattice panel?
body: <p>Consider this example</p>
  
  <pre><code>library(lubridate)
  library(tibble)
  library(lattice)
  
  dat &lt;- tibble(mydate = c(ymd('2018-02-01'),
                    ymd('2018-02-02'),
                    ymd('2018-02-03'),
                    ymd('2018-02-04'),
                    ymd('2018-02-05')),
         myx = c(1,2,3,4,5),
         myz = c(2,3,2,3,1))
  
  # A tibble: 5 x 3
    mydate       myx   myz
    &lt;date&gt;     &lt;dbl&gt; &lt;dbl&gt;
  1 2018-02-01     1     2
  2 2018-02-02     2     3
  3 2018-02-03     3     2
  4 2018-02-04     4     3
  5 2018-02-05     5     1
  </code></pre>
  
  <p>I know I can easily plot the two time series using <code>xyplot</code></p>
  
  <pre><code>dat %&gt;% xyplot(myx + myz ~ mydate, outer = TRUE, data = ., type = 'l',
                 layout = c(1,2))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/3LSiu.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/3LSiu.png" alt="enter image description here"></a></p>
  
  <p>My problem is that I have another <code>tibble</code> that contains dates of interest and I would like to plot vertical lines in the chart above for these dates.</p>
  
  <pre><code>events &lt;- tibble(mydate = c(ymd('2018-02-03'),
                              ymd('2018-02-05')))
  </code></pre>
  
  <p>How can I do that easily?
  Thanks!</p>