plotting x-axes with custom label in R - r

Question

I've to plot these data:
day temperature
02/01/2012 13:30:00 10
10/01/2012 20:30:00 8
15/01/2012 13:30:00 12
25/01/2012 20:30:00 6
02/02/2012 13:30:00 5
10/02/2012 20:30:00 3
15/02/2012 13:30:00 6
25/02/2012 20:30:00 -1
02/03/2012 13:30:00 4
10/03/2012 20:30:00 -2
15/03/2012 13:30:00 7
25/03/2012 20:30:00 1
in the x-axis I want to label only the month and the day (e.g. Jan 02 ). How can I do this using the command plot() and axis()?

Answer 1

First, you will need to put your date text into a dtae class (e.g. as.POSIXct):
df <- structure(list(day = structure(list(sec = c(0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0), min = c(30L, 30L, 30L, 30L, 30L, 30L, 30L,
30L, 30L, 30L, 30L, 30L), hour = c(13L, 20L, 13L, 20L, 13L, 20L,
13L, 20L, 13L, 20L, 13L, 20L), mday = c(2L, 10L, 15L, 25L, 2L,
10L, 15L, 25L, 2L, 10L, 15L, 25L), mon = c(0L, 0L, 0L, 0L, 1L,
1L, 1L, 1L, 2L, 2L, 2L, 2L), year = c(112L, 112L, 112L, 112L,
112L, 112L, 112L, 112L, 112L, 112L, 112L, 112L), wday = c(1L,
2L, 0L, 3L, 4L, 5L, 3L, 6L, 5L, 6L, 4L, 0L), yday = c(1L, 9L,
14L, 24L, 32L, 40L, 45L, 55L, 61L, 69L, 74L, 84L), isdst = c(0L,
0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 1L)), .Names = c("sec",
"min", "hour", "mday", "mon", "year", "wday", "yday", "isdst"
), class = c("POSIXlt", "POSIXt")), temperature = c(10L, 8L,
12L, 6L, 5L, 3L, 6L, -1L, 4L, -2L, 7L, 1L)), .Names = c("day",
"temperature"), row.names = c(NA, -12L), class = "data.frame")
df
df$day <- as.POSIXct(df$day, format="%d/%m/%Y %H:%M:%S")
Your dates should now plot correctly. Don't apply the x-axis, by using the argument xaxt="n". Afterwards, you can create a sequence of dates where you would like your axis labeled, and apply this with axis.POSIXct:
plot(df$day, df$temperature, t="l", ylab="Temperature", xlab="Date", xaxt="n")
SEQ <- seq(min(df$day), max(df$day), by="months")
axis.POSIXct(SEQ, at=SEQ, side=1, format="%b %Y")
Similarly, to get a daily axis, simply modify the SEQ and axis.POSIXct code accordingly. For example, you may try:
plot(df$day, df$temperature, t="l", ylab="Temperature", xlab="Date", xaxt="n")
SEQ <- seq(min(df$day), max(df$day), by="days")
axis.POSIXct(SEQ, at=SEQ, side=1, format="%b %d")