增加ggplot2中日期的轴刻度数

Question

I have some time series data to plot and I'd like more granular axis ticks. For example:

library(lubridate)
library(ggplot2)
library(scales)

dat <- data.frame(date = paste0("09-01-", gsub(" ", "0", format(1:30))),
                  stringsAsFactors = FALSE)
dat$date <- ymd(dat$date)
dat$y <- rnorm(nrow(dat))

p <- ggplot(dat, aes(x = date, y = y)) + geom_point()

Based on this post I tried

p  + scale_x_continuous(breaks=pretty_breaks(n=10))

but Error: Discrete value supplied to continuous scale . Any ideas on how to do this?

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  

Answer 1

Try changing the class of your date field and using scale_x_date :

dat$date <- as.Date(dat$date)
p <- ggplot(dat, aes(x = date, y = y)) + geom_point()
p + scale_x_date(breaks = pretty_breaks(10))