I made a plot with the data you posted where the x axis is in H:M:S and I added a red vertical line. I used the hms package to change your Sunset_Sunrise column from a factor to a numeric difftime value which is easier to handle in plots. I also added a Dummy column for y values since none of the available columns was convenient for plotting a line plot. I hope this gets you started. If you are still stuck, please provide the specific ggplot code and data.
library(ggplot2)
library(hms)
DF <- structure(list(Plot = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 2L,
2L, 2L, 2L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 4L, 4L, 4L),
.Label = c("ALP","BEA", "BET", "CEL", "CHI", "DAV", "ISO",
"JUS", "LAE", "LAU", "LEN", "NAT", "NEU", "NOV",
"OTH", "SCH", "VIS", "VOR"), class = "factor"),
Date = structure(c(88L, 60L, 9L, 2L, 49L, 50L, 75L, 61L,
35L, 90L, 54L, 87L, 3L, 94L, 30L, 13L, 47L, 14L, 79L, 86L
), .Label = c("01.07.21", "01.08.11", "02.07.01", "02.08.11",
"02.08.21", "02.09.11", "03.05.96", "03.08.01", "03.08.04",
"03.08.21", "04.06.96", "04.07.01", "04.07.11", "04.08.97",
"05.07.21", "05.08.96", "06.07.01", "06.08.04", "06.08.21",
"07.03.11", "08.04.97", "08.05.96", "08.07.11", "08.07.19",
"08.07.97", "08.08.19", "09.07.13", "09.07.21", "09.07.97",
"09.08.04", "09.08.11", "09.08.21", "10.03.11", "10.07.01",
"10.07.21", "10.08.11", "10.08.21", "11.06.96", "11.07.04",
"11.07.11", "11.07.21", "11.08.08", "11.08.11", "11.08.21",
"12.07.04", "12.07.11", "12.07.21", "12.08.01", "12.08.11",
"12.08.21", "13.07.04", "13.08.11", "13.08.19", "14.05.96",
"14.08.21", "15.07.97", "15.08.01", "15.08.04", "15.08.96",
"16.08.01", "16.08.11", "16.12.21", "17.08.04", "17.08.11",
"18.08.11", "18.08.21", "19.08.21", "20.07.11", "21.07.01",
"21.07.04", "21.07.97", "22.07.01", "22.07.04", "22.07.97",
"22.08.01", "22.08.11", "23.07.96", "23.08.01", "24.07.01",
"24.07.96", "25.06.21", "26.07.01", "27.06.11", "27.07.01",
"28.06.11", "28.07.04", "28.07.97", "29.05.96", "29.07.04",
"29.07.21", "29.08.11", "30.06.04", "30.08.04", "31.07.01",
"31.07.21"), class = "factor"),
Sunset_Sunrise = structure(c(64L,35L, 49L, 7L, 41L, 13L, 30L, 36L, 77L, 57L, 54L, 62L, 1L,
8L, 47L, 88L, 3L, 45L, 4L, 52L),
.Label = c("", "05:33:52","05:45:00", "05:51:51", "06:00:00", "06:00:14", "06:05:00",
"06:07:35", "06:09:00", "06:16:17", "06:17:09", "06:17:58",
"06:18:25", "06:18:48", "06:19:03", "06:22:03", "06:23:00",
"06:35:00", "06:36:00", "06:59:00", "16:49:23", "18:22:00",
"19:57:00", "20:09:00", "20:10:52", "20:12:44", "20:15:17",
"20:23:00", "20:24:00", "20:27:02", "20:29:10", "20:29:37",
"20:34:00", "20:34:03", "20:34:31", "20:37:00", "20:38:00",
"20:40:00", "20:40:03", "20:40:07", "20:41:00", "20:42:42",
"20:43:00", "20:45:00", "20:47:22", "20:48:05", "20:51:17",
"20:52:09", "20:54:00", "20:54:14", "20:54:15", "20:56:18",
"20:57:00", "20:58:46", "21:01:49", "21:03:08", "21:04:00",
"21:05:02", "21:07:00", "21:07:36", "21:08:51", "21:08:56",
"21:09:50", "21:09:57", "21:10:06", "21:10:35", "21:11:39",
"21:13:56", "21:14:44", "21:18:00", "21:18:32", "21:19:38",
"21:19:55", "21:20:00", "21:20:05", "21:20:45", "21:22:00",
"21:23:00", "21:23:13", "21:24:30", "21:24:40", "21:25:00",
"21:25:06", "21:25:40", "21:25:50", "21:26:00", "21:26:30",
"21:27:00", "21:27:05", "21:27:37", "21:28:00", "21:30:00"),
class = "factor"),
Sky = structure(c(1L, 1L, 1L, 1L, 4L,3L, 1L, 3L, 8L, 3L, 1L, 1L, 1L, 1L, 1L, 8L, 6L, 1L, 1L, 1L),
.Label = c("", "almost cloudless", "cloudless", "cloudy", "cloudy (check time sunset)",
"first cloudless, then partly cloudy",
"overcast", "partly cloudy", "partly cloudy horizon ", "very cloudy"
), class = "factor")), row.names = c(NA, 20L), class = "data.frame")
#next 3 lines change Sunset_Sunrise to a numeric value
DF$Sunset_Sunrise <- as.character(DF$Sunset_Sunrise)
#as_hms does not like the row with the empty string, substitute with NA
DF$Sunset_Sunrise <- ifelse(DF$Sunset_Sunrise=="",NA,DF$Sunset_Sunrise)
DF$Sunset_Sunrise <- as_hms(DF$Sunset_Sunrise)
DF$Dummy <- 1:20 #added a column to use as the y axis values
ggplot(DF,aes(x=Sunset_Sunrise,y=Dummy)) + geom_line()+
geom_vline(xintercept=as_hms("06:31:23"),color="red")
#> Warning: Removed 1 row(s) containing missing values (geom_path).
Created on 2022-01-09 by the reprex package (v2.0.1)