R：ggplot2 x轴作为连续体打印，但需要作为离散值

My data:

我的数据：

structure(list(YEAR = structure(1:10, .Label = c("1980", "1981", 
"1982", "1983", "1984", "1985", "1986", "1987", "1988", "1989"
), class = "factor"), lupas = c(1185, 822, 1340, 853, 3018, 1625, 
966, 1505, 1085, 1754)), .Names = c("YEAR", "lupas"), row.names = c(NA, 
-10L), class = c("tbl_df", "tbl", "data.frame"), drop = TRUE)
structure(list(YEAR = struct