ggplot2 plot function and strptime(as.character() )

Question

I have a time series data and I 'want to plot it by using this code:

temp  <- read.csv("C:/Thesis/Data_set/grosseroor/25.csv",
                        header = FALSE)

names(temp) <- c("dt","ambtemp")
require(ggplot2)
library(scales)
temp$dt <-strptime(as.character(temp$dt), format = "%Y/%m/%d %H:%M")
ggplot(temp, aes(dt, ambtemp)) + geom_line() +
  scale_x_datetime(breaks = date_breaks("5 hour"),labels=date_format("%H:%M")) +
  xlab("Time 00.00 ~ 24:00 (2007-09-29)") + ylab("Tempreture")

Sample data:

date_time                
    10/20/2007 4:52 -6.14
    10/20/2007 4:54 -6.17
    10/20/2007 4:56 -6.09
    10/20/2007 5:00 -7.2
    10/20/2007 5:02 -6.65
    10/20/2007 5:04 -6.04
    10/20/2007 5:16 -6.26
    10/20/2007 5:20 -6.52
    10/20/2007 5:22 -6.4
    10/20/2007 5:24 -6.92
    10/20/2007 5:26 -7.04
    10/20/2007 5:28 -6.84
    10/20/2007 5:30 -6.16
    10/20/2007 5:32 -7.13
    10/20/2007 5:34 -7.43
    10/20/2007 5:36 -6.84
    10/20/2007 5:38 -7.82
    10/20/2007 5:40 -6.84
    10/20/2007 5:42 -6.84
    10/20/2007 5:44 -7.46
    10/20/2007 5:46 -7.74
    10/20/2007 5:48 -8.04

But when I ran the line temp$dt <-strptime((temp$dt), format = "%Y/%m/%d %H:%M"), all the date-time column changed to NA.

@SvenHohenstein, dim(temp) [1] 1367 2 and 'data.frame': 1367 obs. of 2 variables: $ dt : POSIXlt, format: NA NA NA NA ... $ ambtemp: Factor w/ 332 levels "-10 2","-10.02",..: 71 74 67 158 116 62 83 105 94 137 ... — Hamed Footohi
– Hamed Footohi, Commented Jan 28, 2013 at 17:47

Sven Hohenstein · Accepted Answer · 2013-01-29 07:33:14Z

The problem is due to a wrong format string in strptime. You specified %m/%d/%Y, but the data is formatted in a different way: month/day/year. Hence, you have to change the command to:

temp$dt <-strptime(as.character(temp$dt), format = "%m/%d/%Y %H:%M")

As you will see, the order of the date elements then automatically appears as year - month - day: 2007-10-20.

There's another issue with the column ambtemp. As one can see in your comment, it's a factor. I assume it should be numeric. You can transform it the with the following command:

temp$ambtemp <- as.numeric(as.character(temp$ambtemp ))

Now, you could plot the data (I did not change the following commands):

require(ggplot2)
library(scales)

ggplot(temp, aes(dt, ambtemp)) + geom_line() +
  scale_x_datetime(breaks = date_breaks("5 hour"),labels=date_format("%H:%M")) + 
  xlab("Time 00.00 ~ 24:00 (2007-09-29)") + 
  ylab("Tempreture")

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