R用ggplot2表达式给出奇怪的错误:没有找到对象的rversion。

I encountered this error in Ubuntu 14.04 64-bit, Rstudio 0.97.551 , R 3.1.0. I suspect this is an error that has to do with the system environment or rstudio, but I am providing all the details just in case. I am plotting a panel of plots using facet_grid. I have an irreproducible error when I use the following expression using ggplot2 I encountered this error in Ubuntu 14.04 64-bit