title: Plotting the occurrence of an item per date
body: <p>I have this kind of data:</p>
  
  <pre><code>dat
       date shop_id
  1 2013-01       1
  2 2013-02       2
  3 2013-02       2
  4 2013-02       2
  5 2013-02       1
  6 2013-03       3
  7 2013-04       1
  </code></pre>
  
  <p><code>shop_id</code> stands for a specific shop and <code>year_month</code> stands for the date. If a shop is listed at a specific date it means that it´s open, if not it´s closed (i.e. in Janury 2013/2013-01 shop 1 was open but not shop 2 and 3, in March 2013/2013-03 shop 3 was open but not shop 1 and 2). Since the data is about sales of a specific product, a shop can occur more than once per date. I want to plot the data. 
  It should look like the plot below: On the y-axis should be the date, on the x-axis should be the shop_id and fill should be if the shop is open (shop_id occurs together with a specific date) or not. </p>
  
  <pre><code>  dput(dat)
      structure(list(date = structure(c(1L, 2L, 2L, 2L, 2L, 3L, 4L), .Label = c("2013-01", 
  "2013-02", "2013-03", "2013-04"), class = "factor"), shop_id = c(1, 
  2, 2, 2, 1, 3, 1)), class = "data.frame", row.names = c(NA, -7L
  ))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/MFW32.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/MFW32.png" alt="enter image description here"></a></p>