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)