title: Formatting a scale_x_continuous axis with quarterly data
body: <p>I have a data set of counted things, in two groups, aggregated to quarterly counts. The <code>Date_Qtr</code> variable was derived from a larger data set with lubridate. The data frame is as follows.</p>
  
  <pre><code>dat = structure(list(Group = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 
  1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 
  2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L), .Label = c("TypeA", 
  "TypeB"), class = "factor"), Date_Qtr = c(2011.1, 2011.2, 2011.3, 
  2011.4, 2012.1, 2012.2, 2012.3, 2012.4, 2013.1, 2013.2, 2013.3, 
  2013.4, 2014.1, 2014.2, 2014.3, 2014.4, 2015.1, 2015.2, 2011.1, 
  2011.2, 2011.3, 2011.4, 2012.1, 2012.2, 2012.3, 2012.4, 2013.1, 
  2013.2, 2013.3, 2013.4, 2014.1, 2014.2, 2014.3, 2014.4, 2015.1, 
  2015.2), Counts = c(105L, 82L, 72L, 79L, 93L, 118L, 81L, 96L, 
  84L, 83L, 84L, 81L, 99L, 103L, 111L, 80L, 127L, 107L, 54L, 51L, 
  64L, 64L, 53L, 65L, 78L, 63L, 92L, 61L, 80L, 71L, 88L, 66L, 67L, 
  57L, 75L, 59L)), .Names = c("Group", "Date_Qtr", "Counts"), class = "data.frame", row.names = c(NA, 
  -36L))
  </code></pre>
  
  <p>I have plotted a time series in ggplot2 as follows, with the Date_Qtr variable as a <code>scale_x_continuous</code>. Formerly, when I plotted monthly data it was easy to assign breaks at quartely intervals.</p>
  
  <pre><code>ggplot(dat, aes(x = Date_Qtr, y = Counts)) + 
    geom_point( aes( color = Group ), size = 3) + 
    geom_line(aes(color = Group), size = 0.8) +
    scale_y_continuous("Number of things", 
                       limits = c(0, 150)) +
    scale_x_continuous("Year and quarter when things were counted") +
    theme_bw() + 
    theme(axis.text.x = element_text(angle = 45, vjust = 0.5),
          legend.title = element_blank(),
          legend.position = c(0.4, 0.85))
  </code></pre>
  
  <p>Is it possible, with a continuous scale, to represent the data as the actual quarters for each data point, preferably in a format "Jan-Mar 2012" etc.</p>
  
  <p>Thanks in advance.</p>