title: Weekdays below date on x-axis in ggplot2
body: <p>I have a data set with dates plotted along the x-axis (see full dataset below):</p>
  
  <pre><code>&gt; head(test)
          date variable value
  1 2018-01-17      foo    14
  2 2018-01-18      foo    18
  3 2018-01-19      foo    22
  4 2018-01-20      foo    15
  5 2018-01-21      foo    32
  6 2018-01-22      foo    27
  </code></pre>
  
  <p>My goal is to include a second row of x-axis labels (with day of week) below the current one which shows just the date. </p>
  
  <p>However, when I include an annotation in ggplot, this throws an error.  Could anyone suggest a workaround? </p>
  
  <pre><code>ggplot(data=test,
         aes(x=date, y=value, colour=variable)) +
         geom_line() + geom_text(aes(label=value),hjust=0, vjust=0) + 
        annotate(geom = "text", x=date, y= -2, label = weekdays(test$date), size = 4, angle = 90)
  
  Error in (function (..., row.names = NULL, check.rows = FALSE, check.names = TRUE,  : 
    arguments imply differing number of rows: 0, 1
  </code></pre>
  
  <p>Here is the sample data:</p>
  
  <pre><code>&gt; dput(test)
  structure(list(date = structure(c(17548, 17549, 17550, 17551, 
  17552, 17553, 17554, 17555, 17556, 17557, 17558, 17548, 17549, 
  17550, 17551, 17552, 17553, 17554, 17555, 17556, 17557, 17558
  ), class = "Date"), variable = c("foo", "foo", "foo", "foo", 
  "foo", "foo", "foo", "foo", "foo", "foo", "foo", "bar", "bar", 
  "bar", "bar", "bar", "bar", "bar", "bar", "bar", "bar", "bar"
  ), value = c(14L, 18L, 22L, 15L, 32L, 27L, 6L, 3L, 2L, 10L, 23L, 
  16L, 22L, 28L, 31L, 56L, 57L, 23L, 17L, 12L, 33L, 54L)), row.names = c(NA, 
  -22L), .Names = c("date", "variable", "value"), class = "data.frame")
  </code></pre>