ggplot2作图详解2：ggplot图形对象

## R version 3.1.0 (2014-04-10)## Platform: x86_64-pc-linux-gnu (64-bit)## ## locale:##  [1] LC_CTYPE=zh_CN.UTF-8       LC_NUMERIC=C              ##  [3] LC_TIME=zh_CN.UTF-8        LC_COLLATE=zh_CN.UTF-8    ##  [5] LC_MONETARY=zh_CN.UTF-8    LC_MESSAGES=zh_CN.UTF-8   ##  [7] LC_PAPER=zh_CN.UTF-8       LC_NAME=C                 ##  [9] LC_ADDRESS=C               LC_TELEPHONE=C            ## [11] LC_MEASUREMENT=zh_CN.UTF-8 LC_IDENTIFICATION=C       ## ## attached base packages:## [1] tcltk     stats     graphics  grDevices utils     datasets  methods  ## [8] base     ## ## other attached packages:## [1] ggplot2_0.9.3.1 zblog_0.1.0     knitr_1.5      ## ## loaded via a namespace (and not attached):##  [1] colorspace_1.2-4 digest_0.6.4     evaluate_0.5.3   formatR_0.10    ##  [5] grid_3.1.0       gtable_0.1.2     highr_0.3        labeling_0.2    ##  [9] MASS_7.3-31      munsell_0.4.2    plyr_1.8.1       proto_0.3-10    ## [13] Rcpp_0.11.1      reshape2_1.2.2   scales_0.2.4     stringr_0.6.2   ## [17] tools_3.1.0