title: ggplot2 scale x date?
body: <p>I would like to make the ticks in the following graph daily instead of every 2 days.</p>
  
  <pre><code>df1 &lt;- structure(
    list(
      Timestamp = structure(
        c(
          1441837436, 1441843661,
          1441885583, 1441966341, 1441985621, 1442048926, 1442321691, 1442329081,
          1442349761, 1442408140, 1442417679, 1442508871, 1442513339, 1442513395,
          1442514010, 1442525088, 1442553226, 1442562304
        ), tzone = "UTC", class = c("POSIXct",
                                    "POSIXt")
      ), number = 7:24
    ), class = "data.frame", row.names = c(NA,-18L), .Names = c("Timestamp", "number")
  )
  
  ggplot(df1, aes(x = Timestamp, y = number)) +
    geom_line(size=2) + geom_point(size=5) + 
    scale_y_continuous(breaks = seq(0, 50, by = 2))
  </code></pre>
  
  <p>I tried adding <code>+ scale_x_date(breaks = "1 day")</code> but I get the following error:</p>
  
  <pre><code>Error: Invalid input: date_trans works with objects of class Date only
  </code></pre>
  
  <p>I also tried <code>scale_x_datetime(limits=c(as.POSIXct('2015-09-09'), as.POSIXct('2015-09-19')), breaks = 1)</code></p>
  
  <hr>
  
  <pre><code>ggplot(df1, aes(x = Timestamp, y = number)) +
    geom_line(size=2) + geom_point(size=5) + scale_x_datetime(breaks = "1 day")
  Error in strsplit(unitspec, " ") : non-character argument
  </code></pre>
  
  <p>does not work either :(</p>
  
  <hr>
  
  <p>I tried in two different computers:</p>
  
  <pre><code>&gt; sessionInfo()
  R version 3.2.2 (2015-08-14)
  Platform: x86_64-pc-linux-gnu (64-bit)
  Running under: Debian GNU/Linux 8 (jessie)
  
  locale:
   [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
   [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
   [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
   [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                 
   [9] LC_ADDRESS=C               LC_TELEPHONE=C            
  [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets 
  [6] methods   base     
  
  other attached packages:
  [1] ggplot2_1.0.1.9003 taucharts_0.3.2   
  [3] dplyr_0.4.3        fasttime_1.0-1    
  [5] lubridate_1.3.3    readr_0.1.1       
  
  loaded via a namespace (and not attached):
   [1] Rcpp_0.12.1        digest_0.6.8      
   [3] assertthat_0.1     grid_3.2.2        
   [5] plyr_1.8.3         R6_2.1.1          
   [7] gtable_0.1.2       DBI_0.3.1         
   [9] magrittr_1.5       scales_0.3.0.9000 
  [11] stringi_0.5-5      lazyeval_0.1.10   
  [13] RColorBrewer_1.1-2 tools_3.2.2       
  [15] stringr_1.0.0      htmlwidgets_0.5   
  [17] munsell_0.4.2      parallel_3.2.2    
  [19] colorspace_1.2-6   htmltools_0.2.6   
  [21] memoise_0.2.1   
  </code></pre>
  
  <p>2.</p>
  
  <pre><code>&gt; sessionInfo()
  R version 3.2.1 (2015-06-18)
  Platform: x86_64-w64-mingw32/x64 (64-bit)
  Running under: Windows 7 x64 (build 7601) Service Pack 1
  
  locale:
  [1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United States.1252    LC_MONETARY=English_United States.1252
  [4] LC_NUMERIC=C                           LC_TIME=English_United States.1252    
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  other attached packages:
  [1] dplyr_0.4.3        ggplot2_1.0.1.9003
  
  loaded via a namespace (and not attached):
   [1] colorspace_1.2-6  scales_0.3.0.9000 lazyeval_0.1.10   magrittr_1.5      R6_2.1.1          assertthat_0.1   
   [7] plyr_1.8.3        parallel_3.2.1    DBI_0.3.1         tools_3.2.1       gtable_0.1.2      Rcpp_0.12.1      
  [13] grid_3.2.1        munsell_0.4.2  
  </code></pre>