title: ggplot does not plot data in the given order
body: <p>I'm trying to plot some date points:</p>
  
  <pre><code>library(ggplot2)
  library(plyr)
  set.seed(1234)
  
  df &lt;- data.frame(
    c1 = as.character(1:10),
    c2 = sample(seq(from=as.Date("2012-01-01"),to=as.Date("2012-12-01"),by="day"), 10)
  )
  df &lt;- arrange(df,c2)
  qplot(c2,c1,data=df) + scale_x_date(breaks="1 month")
  </code></pre>
  
  <p>Although I used <em>arrange</em> to sort the data, ggplot does not plot the data in order:
  <img src="https://i.stack.imgur.com/Egcgz.png" alt="enter image description here"></p>
  
  <p>The expected plot is:</p>
  
  <p><img src="https://i.stack.imgur.com/iQvKQ.png" alt="enter image description here"></p>
  
  <p>How do I tell ggplot to plot the c1 column in the given order rather than the natural sort order for the "character" type ?</p>
  
  <p>SessionInfo():</p>
  
  <pre><code>R version 2.14.1 (2011-12-22)
  Platform: x86_64-pc-linux-gnu (64-bit)
  
  locale:
   [1] LC_CTYPE=fr_FR.UTF-8       LC_NUMERIC=C               LC_TIME=fr_FR.UTF-8        LC_COLLATE=fr_FR.UTF-8    
   [5] LC_MONETARY=fr_FR.UTF-8    LC_MESSAGES=fr_FR.UTF-8    LC_PAPER=C                 LC_NAME=C                 
   [9] LC_ADDRESS=C               LC_TELEPHONE=C             LC_MEASUREMENT=fr_FR.UTF-8 LC_IDENTIFICATION=C       
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  other attached packages:
  [1] plyr_1.7.1      ggplot2_0.9.2.1
  
  loaded via a namespace (and not attached):
   [1] colorspace_1.2-0   dichromat_1.2-4    digest_0.5.2       grid_2.14.1        gtable_0.1.1       labeling_0.1      
   [7] MASS_7.3-16        memoise_0.1        munsell_0.4        proto_0.3-9.2      RColorBrewer_1.0-5 reshape2_1.2.1    
  [13] scales_0.2.2       stringr_0.6.1      tools_2.14.1      
  </code></pre>
  
  <p>dput(df):</p>
  
  <pre><code>structure(list(c1 = structure(c(8L, 1L, 9L, 2L, 4L, 5L, 3L, 7L, 
  10L, 6L), .Label = c("1", "10", "2", "3", "4", "5", "6", "7", 
  "8", "9"), class = "factor"), c2 = structure(c(15343, 15378, 
  15416, 15508, 15543, 15547, 15548, 15551, 15558, 15625), class = "Date")), .Names = c("c1", 
  "c2"), row.names = c(NA, -10L), class = "data.frame")
  </code></pre>