I have a variable named "Medicine" which has seven values in date format, on the following plot, how can I use a red line to indicate the time when the medicine was taken on x axis?
 
The following is my original plot of blood pressure vs. time. 
plot(time, bloodpressure,xlab="Time",ylab="bPress",main="Time VS Blood Pressure", type="l", xaxt="n")
 r <- as.POSIXct(round(range(Time), "days"))
 axis.POSIXct(1, at=seq(r[1], r[2], by="day"), format="%d-%b")
 
Thanks a lot!




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