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#' Computes attractive axis breaks for date-time data
#'
#' pretty.dates indentifies which unit of time the sub-intervals should be
#' measured in to provide approximately n breaks. It then chooses a "pretty"
#' length for the sub-intervals and sets start and endpoints that 1) span the
#' entire range of the data, and 2) allow the breaks to occur on important
#' date-times (i.e. on the hour, on the first of the month, etc.)
#'
#' @param x a vector of POSIXct, POSIXlt, Date, or chron date-time objects
#' @param n integer value of the desired number of breaks
#' @param ... additional arguments to pass to function
#' @return a vector of date-times that can be used as axis tick marks or bin breaks
#' @keywords dplot utilities chron
#' @export
#' @examples
#' x <- seq.Date(as.Date("2009-08-02"), by = "year", length.out = 2)
#' pretty_dates(x, 12)
pretty_dates <- function(x, n, ...) {
otz <- Sys.getenv("TZ")
if (Sys.getenv("TZ") == "") otz <- "unset"
Sys.setenv(TZ = tz(x[1]))
on.exit(if (otz == "unset") {
Sys.unsetenv("TZ")
} else {
Sys.setenv(TZ = otz)
})
rng <- range(x)
diff <- difftime(rng[2], rng[1], units = "secs")
binunits <- pretty_unit(diff / n)
f <- get(paste("pretty", binunits, sep = "_"), mode = "function")
binlength <- f(diff, n)
start <- pretty_point(min(rng), binunits, binlength)
end <- pretty_point(max(rng), binunits, binlength, start = FALSE)
breaks <- seq.POSIXt(start, end, paste(binlength, binunits))
breaks
}
pretty_unit <- function(x, ...) {
if (x > 3600 * 24 * 365) {
return("year")
}
if (x > 3600 * 24 * 30) {
return("month")
}
if (x > 3600 * 24) {
return("day")
}
if (x > 3600) {
return("hour")
}
if (x > 60) {
return("min")
} else {
return("sec")
}
}
pretty_sec <- function(x, n, ...) {
lengths <- c(1, 2, 5, 10, 15, 30, 60)
fit <- abs(x - lengths * n)
lengths[which.min(fit)]
}
pretty_min <- function(x, n, ...) {
span <- x / 60
lengths <- c(1, 2, 5, 10, 15, 30, 60)
fit <- abs(span - lengths * n)
lengths[which.min(fit)]
}
pretty_hour <- function(x, n, ...) {
span <- x / 3600
lengths <- c(1, 2, 3, 4, 6, 8, 12, 24)
fit <- abs(span - lengths * n)
lengths[which.min(fit)]
}
pretty_day <- function(x, n, ...) {
span <- x / (3600 * 24)
pretty(1:span, n = n)[2]
}
pretty_month <- function(x, n, ...) {
span <- x / (3600 * 24 * 30)
lengths <- c(1, 2, 3, 4, 6, 12)
fit <- abs(span - lengths * n)
lengths[which.min(fit)]
}
pretty_year <- function(x, n, ...) {
span <- x / (3600 * 24 * 365)
pretty(1:span, n = n)[2]
}
pretty_point <- function(x, units, length, start = TRUE, ...) {
x <- as.POSIXct(x)
if (units %in% c("day", "year")) {
if (start) {
return(floor_date(x, units))
} else {
return(ceiling_date(x, units))
}
} else {
floors <- c("sec", "min", "hour", "day", "month", "year")
floorto <- floors[match(units, floors) + 1L]
lower <- floor_date(x, floorto)
upper <- ceiling_date(x, floorto)
points <- seq.POSIXt(lower, upper, paste(length, units))
if (start) {
points <- points[points <= x]
} else {
points <- points[points >= x]
}
fit <- as.duration(x - points)
fit <- abs(as.double(fit, "secs"))
points[which.min(fit)]
}
}
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