title: geom_rect( ) not working over Dates
body: <p>I'm trying to create a shaded bar over a variable that is moving in a time series with Dates on the x-axis.</p>
  
  <p>I am using <code>geom_rect()</code> for this. If the days are numbered on the x-axis I have no issues but when there are Dates on the x-axis I get the following error:</p>
  
  <blockquote>
    <p>Error: Discrete value supplied to continuous scale</p>
  </blockquote>
  
  <p>I have tried to change the xmax from Inf to 100, but nothing seems to work. The data is arranged in this format:</p>
  
  <pre><code>set.seed(333)
  Start.Date &lt;- as.Date("2015-05-01")
  Date &lt;- Start.Date + 0:40 
  variable.1 &lt;- rnorm(n = Date, mean = 720, sd = 300)
  df &lt;- data.frame(Date, variable.1)
  df
  
  library(ggplot2)
  
  ggplot(df, aes(x = Date, y = variable.1)) +
      geom_rect(aes(xmin = 0, xmax = Inf, ymin = 500, ymax = 1100), fill = "blue", alpha = .2) +
      geom_point() + geom_line()
  </code></pre>
  
  <p>Thanks for any help. I'm sure I am missing something</p>