I am trying to plot "time" variable (the time is recoded for three days at 5 minutes interval in the format of 2001-05-14 13:45:00)  VS. blood pressure. My code is the following: 
 
 
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")
 
However the plot only shows three values 14-Apr, 15-Apr and 16 Apr, I would like to have 24 hours marked on the x axis for each day. How can I do it?  Thanks a lot!


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