对ggplot和geom_segment的x轴标签感到困惑

Question

我正在绘制一个使用geom_segment表示事件时间的ggplot图。如何使x轴标签恢复为HH：MM：SS格式？

DB[,.(TM, PF)] %>% ggplot(aes(x = TM, xend = TM)) + 
  geom_segment(y = -Inf, yend = Inf, colour = (DB$PF+1)) +
  scale_x_time(limit = c(min(DB$TM), max(DB$TM)))

#                    TM PF
#1: 2019-09-02 13:32:01  0
#2: 2019-09-02 13:32:01  0
#3: 2019-09-02 13:15:38  0
#4: 2019-09-02 13:15:41  0
#5: 2019-09-02 13:15:34  0

我尝试过geom_segment(..., label = as.factor(DB$TM))，但是却出现了错误Error in f(..., self = self) : Breaks and labels are different lengths。

感谢建议