How to get a vertical geom_vline to an x-axis of class date?

as.numeric works to me

ggplot(data=bmelt)+
  geom_line(aes(x=day,y=value,colour=type),size=0.9)+
  scale_color_manual(labels = c("Observed","Counterfactual"),values = c("1","2"))+
  geom_ribbon(data=ita3,aes(x=day, 
      y=expcumresponse, ymin=exp.cr.ll,ymax=exp.cr.uu),alpha=0.2) +
  labs(title="Italy Confirmed cases",
        y ="# Cases ", x = "Date",color="Output")+
  geom_vline(xintercept = as.numeric(ymd("2020-03-13")), linetype="dashed", 
                color = "blue", size=1.5)+
  theme_minimal()

You could also do geom_vline(xintercept = as.numeric(as.Date("2015-01-01")), linetype=4) if you want the line to stay in place whether or not your date is in the 120th row.

Depending on how you pass your "Dates" column to aes, either as.numeric or as.POSIXct works:

library(ggplot2)

1. using aes(as.Date(Dates),...)

   ggplot(df, aes(as.Date(Dates), value)) +
      geom_line() +
      geom_vline(xintercept = as.numeric(as.Date("2020-11-20")), 
                 color = "red", 
                 lwd = 2) 
   

2. using aes(Dates, ...)

   ggplot(df, aes(Dates, value)) +
      geom_line() +
      geom_vline(xintercept = as.POSIXct(as.Date("2020-11-20")), 
                 color = "red", 
                 lwd = 2) 
   

Try as.numeric(mydata$datefield[120]):

gg + geom_vline(xintercept=as.numeric(mydata$datefield[120]), linetype=4)

A simple test example:

library("ggplot2")

tmp <- data.frame(x=rep(seq(as.Date(0, origin="1970-01-01"),
                            length=36, by="1 month"), 2),
                  y=rnorm(72),
                  category=gl(2,36))

p <- ggplot(tmp, aes(x, y, colour=category)) +
     geom_line() +
     geom_vline(xintercept=as.numeric(tmp$x[c(13, 24)]),
                linetype=4, colour="black")
print(p)