我试图绘制一个简单的折线图，但是每年都有这些奇怪的垂直线（年份是我的x轴）。任何建议将不胜感激。

p1 <- ggplot(data=hdb_table,aes(x=year,y=resale_price,color=flat_type)) +
          geom_line()

dput（head（hdb_table）） enter image description here

structure(list(year = c(2015,2015,2015),date = c("2015-01","2015-01","2015-01"),month_no = c("01","01","01"
),month_name = c("Jan","Jan","Jan"
),region = c("North-East","North-East","North-East"),town = c("ANG MO KIO","ANG MO KIO","ANG MO KIO"),estate_type = c("Mature Estate","Mature Estate","Mature Estate"),flat_type = c("3 ROOM","3 ROOM","3 ROOM"),block = c("174","541","163","446","557","603"),street_name = c("ANG MO KIO AVE 4","ANG MO KIO AVE 10","ANG MO KIO AVE 4","ANG MO KIO AVE 5"),storey_range = c("07 TO 09","01 TO 03","07 TO 09","07 TO 09"
),floor_area_sqm = c(60,68,69,67),flat_model = c("Improved","New Generation","New Generation"),lease_commence_date = c(1986,1981,1980,1979,1980),age = c(33,38,39,40,39),remaining_lease = c(70,65,64,63,64),resale_price = c(255000,275000,285000,290000,290000)),class = c("tbl_df","tbl","data.frame"),row.names= c(NA,-6L))