title: missing yearmon labels using ggplot scale_x_yearmon
body: <p>I have grouped some data by month and year, converted to yearmon using zoo and am now plotting it in ggplot. Does anyone know why one of the ticklabels are missing and there is one for 2018-07, when there is no data for that month? </p>
  
  <p>Example data: </p>
  
  <pre><code>df &lt;-  data.frame(dates = c("2019-01", "2019-02", "2018-08", "2018-09", "2018-10", "2018-11", "2018-12"), values= c(0,1,2,3,4,5,6))
  df$dates &lt;- as.yearmon(df$dates)
  
  ggplot(df, aes(x = dates, y = values)) + 
    geom_bar(position="dodge", stat="identity") +      
    theme_light() +
    xlab('Month') +
    ylab('values')+ 
    scale_x_yearmon(format="%Y %m")
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/YYpFZ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/YYpFZ.png" alt="enter image description here"></a></p>