Andri Signorell Andri Signorell - 2 months ago 19
R Question

How to get axis ticks on a Date axis

For usual axis we can get the tick values with axTicks. If x is a Date, this attempt fails.

x <- sort(sample(seq(as.Date("2000-01-01"), as.Date("2010-12-31"), by=1), 50))
y <- runif(50)
plot(y~x, type="l", col="steelblue")

axTicks(1)
abline(v=axTicks(1))


axTicks returns values, but obviously not the ultimately used ones. How can we get the used ticks?

AEF AEF
Answer

I think the tick positions for the plot are created by axis.Date. To get the positions you can copy the relevant parts of axis.Date in your own function:

dateTicks <- function(x, side = 1) {
##  This functions is almost a copy of axis.Date
    x <- as.Date(x)
    range <- par("usr")[if (side%%2) 
        1L:2L
        else 3:4L]
    range[1L] <- ceiling(range[1L])
    range[2L] <- floor(range[2L])
    d <- range[2L] - range[1L]
    z <- c(range, x[is.finite(x)])
    class(z) <- "Date"
    if (d < 7) 
        format <- "%a"
    if (d < 100) {
        z <- structure(pretty(z), class = "Date")
        format <- "%b %d"
    }
    else if (d < 1.1 * 365) {
        zz <- as.POSIXlt(z)
        zz$mday <- 1
        zz$mon <- pretty(zz$mon)
        m <- length(zz$mon)
        m <- rep.int(zz$year[1L], m)
        zz$year <- c(m, m + 1)
        z <- as.Date(zz)
        format <- "%b"
    }
    else {
        zz <- as.POSIXlt(z)
        zz$mday <- 1
        zz$mon <- 0
        zz$year <- pretty(zz$year)
        z <- as.Date(zz)
        format <- "%Y"
    }
    keep <- z >= range[1L] & z <= range[2L]
    z <- z[keep]
    z <- sort(unique(z))
    class(z) <- "Date"
    z
}

If you in contrast look at the source for axTicks you can see that for dates it boils down to seq.int(axp[1L], axp[2L], length.out = 1L + abs(axp[3L])) with axp = par("xaxp"). So it's just converting the dates to numeric and dividing the resulting range evenly.

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