Error from ggplot2 plotting date data — missing value where TRUE/FALSE needed

Question

I'm trying to plot some grades I'm pulling from an external source. The date format comes in looking like this:

2011-08-23T17:07:05

So I parse it with strptime(date, "%FT%X") and get a POSIXlt. I end up with a complete data frame that looks like this:

                  date    subject  grade
1  2011-08-23 17:07:05 AP Biology  95.83
2  2011-08-24 17:07:03 AP Biology  95.83
3  2011-08-25 17:08:27 AP Biology  95.83
4  2011-08-17 17:05:54 US History 157.14
5  2011-08-18 17:05:24 US History 157.14
6  2011-08-19 17:05:35 US History 157.14
7  2011-08-22 17:06:25 US History 157.14
8  2011-08-23 17:07:05 US History 157.14
9  2011-08-24 17:07:03 US History 157.14
10 2011-08-25 17:08:27 US History 157.14
11 2011-08-19 17:05:35   Yearbook   0.00
12 2011-08-22 17:06:25   Yearbook   0.00
13 2011-08-23 17:07:05   Yearbook 100.00
14 2011-08-24 17:07:03   Yearbook 100.00
15 2011-08-25 17:08:27   Yearbook 100.00

With the following structure:

'data.frame':   15 obs. of  3 variables:
 $ date   : POSIXlt, format: "2011-08-23 17:07:05" "2011-08-24 17:07:03" ...
 $ subject: Factor w/ 3 levels "AP Biology","US History",..: 1 1 1 2 2 2 2 ...
 $ grade  : num  95.8 95.8 95.8 157.1 157.1 ...

When I try to plot this data:

> ggplot(data=grades, aes(date, grade, factor=subject)) + geom_line()
Error in if (length(range) == 1 || diff(range) == 0) { : 
  missing value where TRUE/FALSE needed

I don't know what I'm doing wrong here. I narrowed it down to the date handling by doing this:

ggplot(data=grades,
       aes(seq(length(grades[,1])),
           grade, color=subject)) + geom_line()

... but how do I do the date handling correctly?

Answer 1

Only times of class POSIXct are supported in ggplot2. Class POSIXct represents the (signed) number of seconds since the beginning of 1970 (in the UTC timezone) as a numeric vector. Class POSIXlt is a named list of vectors representing nine elements (sec, min, hour, etc.).

You can use the following:

grades$date <- as.POSIXct(grades$date)

Answer 2

I think I've figured this out. The difference is in understanding POSIXct vs. POSIXlt. POSIXlt is the calendar time in parts. POSIXct is the number of seconds since the epoch. strptime returns a `POSIXct

In order to use this data, I need to convert the timestamps:

grades$date <- as.POSIXct(grades$date)