ggplot2：我无法理解的错误消息

Question

I have a data frame "df1" of the following kind:

set.seed(2)
df1 = data.frame(Var1=rep(c('a','b','c','d'),3),
                Var2=rep(c('aa','bb','cc'),each=4),
                value=rnorm(12))

I try to create this plot:

ggplot(df1) + geom_bar(aes(x=Var2,y=value),stat="identity") + facet_wrap(~Var1)

and I get the following error message that I don't understand:

formal classes cannot be used without the methods package

What's wrong ? I can't find a package called methods by the way.

Here is my sessionInfo()

R version 3.0.1 (2013-05-16)
Platform: x86_64-apple-darwin10.8.0 (64-bit)

locale:
[1] fr_CH.UTF-8/fr_CH.UTF-8/fr_CH.UTF-8/C/fr_CH.UTF-8/fr_CH.UTF-8

attached base packages:
[1] utils    graphics methods  stats    base    

other attached packages:
[1] agricolae_1.1-4  reshape2_1.2.2   lattice_0.20-15  ggplot2_0.9.3.1  data.table_1.8.8

loaded via a namespace (and not attached):
 [1] colorspace_1.2-2   datasets_3.0.1     dichromat_2.0-0    digest_0.6.3       grDevices_3.0.1    grid_3.0.1         gtable_0.1.2      
 [8] labeling_0.1       lme4_0.999999-2    MASS_7.3-26        Matrix_1.0-12      munsell_0.4        nlme_3.1-109       plyr_1.8          
[15] proto_0.3-10       RColorBrewer_1.0-5 scales_0.2.3       stats4_3.0.1       stringr_0.6.2      tools_3.0.1   

Thank you

Answer 1

I encountered the same error message and found a solution but in a different context.

The error happened while running an R script on a cluster. Interestingly the same script worked fine on a laptop (during testing with a subset of the data).

The solution in my context was to import the package with which the data was preprocessed. What I mean is that I previously saved an RDS file created using the package "Seurat". When reading the RDS file in a new session the package "Seurat" would load automatically when ran on a local machine. However on the cluster I had to specifically load the package with the library() command, before reading the RDS file. This is what solved the problem and stopped the obfuscated error message.