title: facet_wrap, facet_grid - using date type in facets
body: <p>Consider the following simple example:</p>
  
  <pre><code>df &lt;- data.frame(
    a = rep(c('a','b','c'),3),
    b = as.Date(rep(c('2011-01-01','2011-02-01','2011-03-01'),3)),
    c = rnorm(9),
    d = rep(1:3,each=3)
    )
  
  str(df)
  
  ggplot(df, aes(x=d, y=c)) + geom_line(aes(group=b)) + facet_wrap(~a)
  
  ggplot(df, aes(x=d, y=c)) + geom_line(aes(group=a)) + facet_wrap(~b)
  </code></pre>
  
  <p>The second ggplot fails with error:</p>
  
  <pre><code>Error in scale_apply(layer_data, x_vars, scale_train, SCALE_X, panel$x_scales) : 
  
  In addition: Warning message:
  In `[&lt;-.factor`(`*tmp*`, rng, value = c(1L, 2L, 3L, 1L, 2L, 3L,  :
    invalid factor level, NA generated
  </code></pre>
  
  <p>When I cast column b (date) to character, it works:</p>
  
  <pre><code>df$b &lt;- as.character(df$b)
  ggplot(df, aes(x=d, y=c)) + geom_line(aes(group=a)) + facet_wrap(~b)
  </code></pre>
  
  <p>I can pass any other data type to <code>facet_wrap</code> except date:</p>
  
  <pre><code>df &lt;- data.frame(
    a = rep(1:3,3),
    b = rep(1:3,each=3),
    c = rnorm(9),
    f = rep(c('a','b','c'),3), 
    g = as.logical(rep(c(TRUE,FALSE,TRUE),3)), 
    h = as.integer(rep(c(1,2,3),3)), 
    i = rep(c(0.5,1.0,1.5),3),
    j = as.factor(rep(c('a','b','c'),3)),
    k = as.complex(rep(c(1,2,3),3)),
    l = as.Date(rep(c('2011-01-01','2011-02-01','2011-03-01'),3))  
    )
  
  str(df)
  
  ggplot(df, aes(x=b, y=c)) + geom_line(aes(group=a)) + facet_wrap(~f) # Character
  ggplot(df, aes(x=b, y=c)) + geom_line(aes(group=a)) + facet_wrap(~g) # Boolean
  ggplot(df, aes(x=b, y=c)) + geom_line(aes(group=a)) + facet_wrap(~h) # Integer
  ggplot(df, aes(x=b, y=c)) + geom_line(aes(group=a)) + facet_wrap(~i) # Numeric
  ggplot(df, aes(x=b, y=c)) + geom_line(aes(group=a)) + facet_wrap(~j) # Factor
  ggplot(df, aes(x=b, y=c)) + geom_line(aes(group=a)) + facet_wrap(~k) # Complex
  ggplot(df, aes(x=b, y=c)) + geom_line(aes(group=a)) + facet_wrap(~l) # Date
  </code></pre>
  
  <p>Is there a way how I can pass date column to <code>facet_wrap</code> or <code>facet_grid</code> without casting the column to character/factor first?</p>
  
  <p>R version 3.0.2 (2013-09-25)
  ggplot2_0.9.3.1</p>