[R] how can i add Qtr1 Qtr2 Qtr3 Qtr4 on the x axis?

[水静流深]

> x<-ts(sales,frequency=4,start=c(2010,1))
> x
     Qtr1 Qtr2 Qtr3 Qtr4
2010  1.8  8.0  6.0  3.0
2011  2.0 11.0  7.0  3.5
2012  2.5 14.0  8.0  4.2
2013  3.0 15.2  9.5  5.0
> plot.ts(x)