Plot dates on the x axis and time on the y axis with ggplot2

Question

I have read in a series of 37 dates and times that an event happened. It is now sitting as a POSIXlt object. I want a graphic representation of the times that the events hap

Answer 1

There are two steps required:

• Extract the time element from the POSIXct object. You can do that with some lubridate extractor functions and a bit of arithmetic, or by subtracting as.Date(dttm) from dttm. I show both ways.
• Add a date-time y-axis and specify a suitable formatter

Two alternative ways to extract the time from a POSIXct object:

dropDate <- function(x){
  3600*hour(x)+60*minute(x)+second(x)
}

dropDate2 <- function(x){
  as.numeric(x - as.Date(x))
}  

You may also wish to specify explicit labels for the axes:

qplot(day(dttm), dropDate(dttm)) + 
    scale_y_datetime(format="%H:%M:%S") + 
    xlab("Day") + ylab("Hour")

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There are more examples of this type of scale in ?scale_datetime, which will point you to ?strptime for an explanation of the date and time formatting codes.

Answer 2

You can tweak the date axes using scale_datetime. The examples at the end are quite illustrative.

http://had.co.nz/ggplot2/scale_datetime.html