Trouble plotting dates with ggplot2 - r

Question

I'm trying to plot against dates in R. I've run into trouble trying to create vertical lines against a plot that I already have. All of the different formats that I try either result in nothing showing up on the plot, or a line at 1970 (the default date). The year-data is in the form yyyy-mm-dd. For example, "1914-07-01".
I've also tried inputting these dates in a data.frame, but got the same problem.
I've been trying to make a reproducible example, but I haven't seen any example datasets to do so with, and got frustrated trying to create one... sorry about that. Here's the relevant code:
ggplot(M,aes(x=date,color=origin,y=value)) +
geom_point() +
geom_line() +
facet_grid(topic~origin) +
geom_vline(xintercept=as.numeric(as.Date("1914-07-01")))
Everything plots correctly without the addition of the final line.
Edit: here's the result of dput(head(M)):
structure(list(topic = structure(c(1L, 1L, 1L, 1L, 1L, 1L), .Label = c("1",
"2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12", "13",
"14", "15", "16", "17", "18", "19", "20", "21", "22", "23", "24",
"25"), class = "factor"), date = structure(c(-1767196800, -1765987200,
-1764518400, -1763308800, -1762099200, -1760889600), class = c("POSIXct",
"POSIXt"), tzone = ""), origin = structure(c(2L, 2L, 2L, 2L,
2L, 2L), .Label = c("Blast", "The_Egoist"), class = "factor"),
value = c(6.69960398194253e-07, 7.48757156068349e-07, 7.04834977806836e-07,
7.10226526475778e-07, 6.8295233938925e-07, 6.16466066169137e-07
)), .Names = c("topic", "date", "origin", "value"), class = c("grouped_df",
"tbl_df", "tbl", "data.frame"), row.names = c(NA, -6L), vars = list(
topic, date), drop = TRUE, indices = list(0L, 1L, 2L, 3L,
4L, 5L), group_sizes = c(1L, 1L, 1L, 1L, 1L, 1L), biggest_group_size = 1L, labels = structure(list(
topic = structure(c(1L, 1L, 1L, 1L, 1L, 1L), .Label = c("1",
"2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12",
"13", "14", "15", "16", "17", "18", "19", "20", "21", "22",
"23", "24", "25"), class = "factor"), date = structure(c(-1767196800,
-1765987200, -1764518400, -1763308800, -1762099200, -1760889600
), class = c("POSIXct", "POSIXt"), tzone = "")), class = "data.frame", row.names = c(NA,
-6L), .Names = c("topic", "date"), vars = list(topic, date)))

Answer 1

You were very close, the problem is your data is in POSIXct, and you were trying to convert to Date. To fix it, change to POSIXct:
ggplot(M,aes(x=date,color=origin,y=value)) +
geom_point() +
geom_line() +
facet_grid(topic~origin) +
geom_vline(xintercept=as.numeric(as.POSIXct("1914-07-01")))
You can see the difference in the calls:
as.numeric(as.POSIXct("1914-07-01"))
[1] -1751569200
as.numeric(as.Date("1914-07-01"))
[1] -20273
Explaining why the intecept was so close to 1970 (the 0 for both)