ggplot2绘制日期数据时出错 - 缺少需要TRUE/FALSE的值

Question

我正在尝试绘制一些我从外部来源提取的成绩.日期格式如下所示:

2011-08-23T17:07:05

所以我解析它strptime(date, "%FT%X")并得到一个POSIXlt.我最终得到了一个完整的数据框,如下所示:

                  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

具有以下结构:

'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 ...

当我尝试绘制这些数据时:

> 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

我不知道我在这里做错了什么.我通过这样做将其缩小到日期处理:

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

...但是如何正确处理日期？

Answer 1

POSIXctggplot2 只支持类的次数.Class POSIXct表示自1970年初(UTC时区)以来的(带符号)秒数作为数字向量.类POSIXlt是表示九个元素(向量的命名列表sec,min,hour等等).

您可以使用以下内容:

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