title: rotating axis labels in date format
body: <p>does anyone know how to rotate axis ticks in the date format with ggplot2?
  I want to get labels with "Date-Month" (ex. "1985-5") with a 45° angle on the x axis.</p>
  
  <p>data sample:</p>
  
  <pre><code>Station Date Ptot
  A 1980-02 16
  A 1980-03 19
  A 1980-04 40
  A 1980-05 48
  A 1980-06 NA
  A 1980-07 18.5
  A 1980-08 24.6
  B 1980-07 50.8
  B 1980-08 28.9
  B 1980-09 32.9
  B 1980-10 47.9
  B 1980-11 16.3
  </code></pre>
  
  <p>I tried this:</p>
  
  <pre><code>library(ggplot2)
  library(scales)
  
  plot &lt;- ggplot(data=na.omit(data), aes(x=Date, y=Ptot, group=Station))+
     geom_line()+
     facet_grid(Station~.)+
     scale_x_date(breaks = "year", labels=date_format("%Y-%m"))+
     xlab("Year")+
     ylab("Prec (mm)")+
     labs(colour = "Station")+
     theme(axis.text.x = element_text(angle = 45, hjust = 1))+
     theme_bw()
  
  plot
  </code></pre>
  
  <p>but it doesn't work.</p>
  
  <p>Thanks!</p>