R/timePal.R

In palr: Colour Palettes for Data

#' Time-indexed colour.
#'
#' Create a time-indexed colour map, useful for maintaining an absolute scale across time series as a function of date-time.
#'
#' @param x date-times
#' @param col colours, can be a function or an actual set of colours
#'
#' @return function of date-time
#' @export
#' @examples
#' dts <- seq(as.Date("1749-01-01"), by = "1 month", length.out = length(sunspots))
#' d <- data.frame(date = dts, sunspots = as.vector(t(sunspots)))
#' tpal <- mk_timePal(d$date, col = sst_pal(50))
#' par(mfrow  = c(2, 1))
#' plot(sunspots ~ date, col = tpal(date), data = d)
#' ## colours maintained by absolute date
#' plot(sunspots ~ date, col = tpal(date), data = d[1500:1800, ], cex = 2)
#' ## we can now insert new points and maintain this colour ramp
#' d2 <- data.frame(date = seq(min(d$date), max(d$date), by = "5 days"))
#' d2$sunspots <- approxfun(d$date, d$sunspots)(d2$date)
#' points(sunspots ~ date, col = tpal(date), data = d2, pch = 19, cex = 0.5)
mk_timePal <- function(x, col) {

  timerange <- range(.datetimeFrom(x))

  if (is.function(col)) {
    cols <- col(256)
  } else {
    cols <- col
  }
  timemap <- seq(timerange[1], timerange[2], length = length(cols) + 1)
  function(x) {
    cols[findInterval(.datetimeFrom(x), timemap)]
  }
}