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

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)

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.

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()

code output