title: ggplot2: override scale_x_datetime with custom month labels
body: <p>Here is a bit of my data:</p>
  
  <pre><code>df &lt;- structure(list(farm = c("F1", "F1", 
  "F1", "F1"), index = structure(c(1L, 
  1L, 4L, 4L), .Label = c("HT", "Mid-T", "Outside Mid-T", "Outside South"
  ), class = "factor"), sensorheight = c(1L, 1L, 1L, 1L), yrmonth = structure(c(1491004800, 
  1496275200, 1491004800, 1496275200), class = c("POSIXct", "POSIXt"
  ), tzone = "UTC"), monthindex = structure(c(2L, 2L, 2L, 2L), .Label = c("Spring", 
  "Winter"), class = "factor"), N = c(2, 2, 1, 1), TempC = c(2.06446759259259, 
  6.68402777777778, 1.32268518518518, 5.63194444444445), sd = c(1.17081824208967, 
  0.034373246307681, NA, NA), se = c(0.827893518518518, 0.0243055555555567, 
  NA, NA), ci = c(10.5193845460483, 0.308831365115372, NA, NA)), .Names = c("farm", 
  "index", "sensorheight", "yrmonth", "monthindex", "N", "TempC", 
  "sd", "se", "ci"), row.names = c(7L, 9L, 20L, 22L), class = "data.frame")
  </code></pre>
  
  <p>I want to graph it with custom month labels on the x-axis:</p>
  
  <pre><code>labels &lt;- c('March*', 'April', 'May*', 'June', 'July')
  </code></pre>
  
  <p>When I add the labels it doesn't put the labels in the correct order:</p>
  
  <pre><code>ggplot(df, aes(x=yrmonth,y=TempC)) +    
     geom_line(size=.4) +    
     geom_point(size=1.5) +      
     scale_x_datetime(name = " ", date_breaks = "1 month", date_labels = labels) +   
     scale_y_continuous(name = "Monthly Average Maximum Temperature at 1 m (°C)")
  </code></pre>