title: Increase number of axis ticks in ggplot2 for dates
body: <p>I have some time series data to plot and I'd like more granular axis ticks. For example:</p>
  
  <pre><code>library(lubridate)
  library(ggplot2)
  library(scales)
  
  dat &lt;- data.frame(date = paste0("09-01-", gsub(" ", "0", format(1:30))),
                    stringsAsFactors = FALSE)
  dat$date &lt;- ymd(dat$date)
  dat$y &lt;- rnorm(nrow(dat))
  
  p &lt;- ggplot(dat, aes(x = date, y = y)) + geom_point()
  </code></pre>
  
  <p>Based on <a href="https://stackoverflow.com/questions/11335836/increase-number-of-axis-ticks-in-ggplot2">this post</a> I tried</p>
  
  <pre><code>p  + scale_x_continuous(breaks=pretty_breaks(n=10))
  </code></pre>
  
  <p>but <code>Error: Discrete value supplied to continuous scale</code>. Any ideas on how to do this? </p>
  
  <pre><code>R Under development (unstable) (2014-12-29 r67265)
  Platform: x86_64-apple-darwin10.8.0 (64-bit)
  Running under: OS X 10.9.5 (Mavericks)
  
  locale:
  [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  other attached packages:
  [1] scales_0.2.4    ggplot2_1.0.0   lubridate_1.3.0
  
  loaded via a namespace (and not attached):
   [1] colorspace_1.2-4 digest_0.6.4     grid_3.2.0       gtable_0.1.2    
   [5] labeling_0.2     MASS_7.3-35      memoise_0.2.1    munsell_0.4.2   
   [9] plyr_1.8.1       proto_0.3-10     Rcpp_0.11.2      reshape2_1.4    
  [13] stringr_0.6.2    tools_3.2.0  
  </code></pre>