I've never had any issues with the way that plot.POSIXct chooses the
labels of the date axis before, but in this particular case it's output
is a little confusing.

plot(seq(as.POSIXct("1997-10-01"),length.out=108,by="month"),rnorm(108))

This command produces a chart with every x tick mark labeled as "Jan
01".

I can replicate this chart by adding the format string "%b %d".

plot(seq(as.POSIXct("1997-10-01"),length.out=108,by="month"),rnorm(108),
format="%b-%d")

My contention is that plot.POSIXct should choose "%Y" in this case,
which would show unique labels rather than repeated labels.

plot(seq(as.POSIXct("1997-10-01"),length.out=108,by="month"),rnorm(108),
format="%Y")



Now, just looking at axis.POSIXct it seems that the terminal condition
of this if/else sequence sets format to "%b %d":

 }
    else if (d < 7 * 60 * 60 * 24) {
        sc <- 60 * 60 * 24
        if (missing(format)) 
            format <- "%a"
    }
    else {
        sc <- 60 * 60 * 24
        if (missing(format)) 
            format <- "%b %d"  #****** here
    }


when the second set of if conditions fires up, format has already bet
set, so the missing(format) test fails here and does not set the format
to %Y:

 else {
        class(z) <- c("POSIXt", "POSIXct")
        zz <- as.POSIXlt(z)
        zz$mday <- 1
        zz$isdst <- zz$mon <- zz$hour <- zz$min <- zz$sec <- 0
        zz$year <- pretty(zz$year)
        z <- as.POSIXct(zz)
        if (missing(format)) 
            format <- "%Y"   #******  here
    }

I'm not sure if this is the intended behavior or not.  Does anyone else
have any thoughts on this issue?

Thanks,
Whit





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